if you wan't to add it as a new planet make a mesh where you have a mesh point which is a flair with the plasma effect. I am not that experienced with planets, so there might be better advices.

Then I can give a more detailed description. You can add a mesh point by openning a .mesh file in text format (it is in bin like entity files and converted the same way) . As an example we can use the colonizeable asteroid.

first copy the AsteroidBig00.mesh and rename it to what you want then open it with notepad and add this point:

Point
  DataString "Flair-PlanetPlasmaStorm"
  Position [ 0.000000 0.000000 0.000000 ]
  Orientation
    [ 1.000000  0.000000  0.000000 ]
    [ 0.000000 1.000000  0.000000 ]
    [ 1.000000  0.000000  1.000000 ]

and add 1 to the NumPoints

then make a new .entity fx by copying the PlanetAsteroid.entity and change AsteroidBig00 in this line:

 meshName "AsteroidBig00"

With the name of the new mesh file you made.

I hope this helped