You can run a set of woofers full range without damaging them. Most factory components do just that. It won't sound as good as it could with a coil on it to cut the high frequencies out, but it will get the job done as fill.
If the choice is between front or rear install, if the front ones have not been replaced, I would put them in the front.
What brand and model are they? If no amp is being used and they have lower spl sensitivity compared to the factory ones, the factory ones may be better back there.
Factory speakers I'm guessing have a solid 91-92 db sensitivity.
Our rear doors, do not deliver very good bass even with good speakers, our front doors get way more bass.