Debris fields can be created the same way as asteroid fields in FRED; however, the documentation on modding it (using custom debris pieces) is pretty foggy.
Actually, the entire implementation is. FRED seems to interpret species_defs.tbl correctly; in ASW for example, the asteroid field editor gives a choice between Ancient Small, Ancient Medium, Ancient Large, Shivan Small etc.
But there doesn't seem to be a way to define which models are actually used for the different species; try making an Ancient debris field and it will use Terran debris models. When targeting a debris piece, it is identified as "Ancient debris" again.
So, uhh, in conclusion: is there a table entry anywhere which defines the models to use for debris fields?