You *could* actually go about doing this, in a very slow, long, drawn out process that would honestly be easy to mess up at any point along the way. You could start, sorta, with the idea of copy/paste and pray, but you'd only wanna do that with non-duplicate files. For all duplicate ones, you'd want to essentially merge them, or pick and choose which to actually use. So it's basically a lot of text editing going to take place. And even if you don't miss something, add a blank line somewhere on accident, or any number of other simple (and they are the most likely to be done, honestly) mistakes, and actually manage to get it all working, there's no telling how long the mod'll run before it dumps from the memory limit. So unfortunately there are a lot of downsides to trying to do this. 'Course, managing will possibly give yourself a very very awesome mod to play with.
If you really DO want to try doing this, I'd probably suggest checking out Beyond Compare, as you can use it to see what's different in all the files/folders of the mods you want to merge. It does cost money, but it's got a 30 day trial that counts only the days of use, not how long it's been installed, so that's nice, and probably long enough for you to get through merging whatever mods you want. I don't know if there's better ones for free, but I know from my own mod-merging for my own amusement that it's pretty nice.
(And of course, remember to ask permission from the mod creators if you want to put it up as DLable for people. I believe most are indifferent to altering for personal use.)
