maybe create a pof for this purpose?
IIRC there are up to ~110 subobjects aviable, so you may create an asteroid field pof with those 110 asteroid. You can make em move (rotating subobjects), which mean to spin or even to move (just put the axis outside the subobject)
If 110 is not enough, you may create big asteroids, one for each subobject, and create smaller ones too, placing one or two of those small asteroids in the same subobject of a big one, but at far distance (so you'll not notice that they explode at the same time if destroyed)
You may also put a regular asteroid field in the same position of your custom asteroidfield.pof
you may also make more than one asteroidfield.pof and place em in the same position (but you'll have to be careful that the single asteroids don't collide with the other asteroids of the same pof and with the asteroids of the other pofs).
The only problem is that IIRC there's also a subobject limit per missions
