A lot of new ideas. You're very creative, but unfortunately even the FreeSpace engine doesn't keep pace. Much of what you want to do could only be done by coding or very cumbersome SEXP workarounds. For example, to go back to your last thread:
2)PCX(whatever image file you use) images apear like the nebular effect (overlayed over each other randomly) but in a set radius.
ANI files are used for explosion effects, and they are purely visual; you can't use them as nebula-asteroids, for example. And getting those missiles to explode where you want them to (assuming that they're being fired from some invisible object) would take a ridiculous ammount of work in FRED.
3)when your ship or an enemy flys into it, it damages there hull a fraction of a % each millisecond there in there.
Again, you could do this in FRED: use every-time/set-object-position to tie a waypoint to each asteroid and have the player take damage any time he nears a waypoint. But it would be very tedious. The only way you could get what you're asking for is to code it in. You may want to look at hazard zones
on the Wiki.
4)other effects could be, it slows down your/enemy ship, EMP, TAG, destroys missiles when fired in to it, ect....
Same as above.
5)the gas starts to disipate after a few seconds, and gets more transparrent and less deadly untill its gone...
Now that would be very complicated. You could use an explosion ANI to show the gas disintegrating, but only after it's been totally destroyed.
If you don't get any replies, it's almost certainl because noone knows how to help you. The short answer to your question is, no, you can't do everything that you wanted to do.
Your best bet is to retexture an asteroid with nebula skins and transparent textures to make it look gas-like, if you really want these things.