I agree with jaxter - the plastic is so thin, and they have no insulation, that it's probably not worth trying (and any heat source might melt the plastic if it's warm enough to actually have an impact and it comes into contact with the plastic).
Edit to add: I've just seen your other thread. I had assumed your thread here was asking in relation to one of those thin transparent plastic mini greenhouses that has 4 shelves and space for 3 seed trays on each. If you've got a more robust greenhouse made out of wood and twin wall polycarbonate, as I think you've mentioned on another thread, then the risk of melting is obviously far less.
But heat is still likely to cost a fortune, and/or could cause lots of moisture and thus mould or damping off risk (e.g., if you went down the paraffin heater route).