You 'could' do it, but there would be only one way of making it appear once in every system, and thats with the Galaxy editor. So a stand alone mod is out of the question. You would just have to put it around a planet, and then have some ability that could capture it. I could write a basic tutorial, but it will be tomorrow as I'm off to bed now (GMT yanks ). Best thing I can suggest is to work on getting it always present around a planet by editing the GalaxyScenarioDef file, and then playing around with abilities, to make it disabled from the start and capturable.

First and foremost, this is all from memory, I'm not about to write down exact names and files for you, I have my own mod to worry about, this is more of a guideline.

Right, I'm going to assume you know how to perform basic modding, because this would otherwise be a very, very long tutorial.

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I'll write here a little 'key' as to what I refer to in this:
Count Number
Above every list in all the files in Sins there is a number of exactly how many items are in that list. Whenever you add to this list, you must add to the count number. This applies for EVERY LIST you edit.

Strings
When you add a new file, you must link its StringID references to the english.str file. So if you create a new planet, or ability, you must update its string ID's and add these to the string file.

GalaxyScenarioDef file
I will refer to this with GSD file.

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OK, firstly, I hope, and if not you should learn, how the GSD file works. So, you would firstly need to create a new planet for this superweapon, and it would be best to copy an existing planet file, and then that planets reference in the GSD file, and just slightly change its file name, maybe have a suffix of -SW, for Super Weapon. For this new planet, you shouldn't need to update the strings as you will probably want it to be an exact copy of its original. Don't forget the count number.

With this new planet, you must add it to the various sections of the GSD file, use a similar approach as the other planets, putting it only once in the top section in its own category, and giving it a custom string that reads something like 'Desert-SuperWeapon'. This string will be viewable in the in-game game map maker.

So, further down (use the search function in notepad, Ctrl+F), and find the planet you copied, then copy it in this other section too. This will have the name of the planet file used for this planet. So you need to point it to the '-SW' file you created earlier. Count number.

Now it gets tricky, and this may be where you decide its too complex. Search in the GSD file for the file name of the superweapon, and find what its called in the GSD file, something like 'Tech:Module:Novalith', it will be displayed like that, with the : separating the various categories it falls in. Now search for the template of the militia forces that guard the planet you copied, usually something like 'Template:DefaultDesertStart'. This will then in turn reference another militia template, like 'Template:Militia:strong' or something to that effect. Ignore that, you will link your planet directly to the one template. Count number of the templates.

So, where you have you planets file name, it will have a template reference, change that to something like 'Template:SW'. Now, go right down to the bottom of the template section, and copy one of the other planets militia forces template. Unfortunately you will have to have the planet owned by militia, so use one of the template whereby it has the militia as its owner, but put it as 'always' so no matter what the map maker options are, it will always be militia owned.

For this test, keep the militia forces, but add the name of the Novalith cannon which we found earlier, which should show like 'Tech:Platform:Novalith' or something. Count number for the items list.

Ok, having done all that, test it. I am almost 100% sure it will not work, and this is where you will have to methodically go through that file, again and again to find the problem, but having followed this rough guide, you should eventually get it.

After you have it working, you will need to make it capturable, but if you only plan on using it for this one purpose of capture, you should be ok as you can add this ability to the Novalith's file itself. That will come once you have done the above.

If that makes no sense to you, you have 2 choices:
1. Give up.
2. Download the modding tutorials and mod tools and get used to modding the basics.

Good luck, you will need it. Oh, and I'd keep a stress ball at hand, or maybe 2.

As an update, no luck getting anything new to show up in the in game map creator, and galaxy forge refuses to load with the new GSD file. Ah well, was worth a shot, I might try it again, but for now, I'm just going to focus on mapping and modeling.