I see no reason why you cannot use the decal features of FPSC to introduce sprites into your game. They will of course require an .x file - though visually I dont see that they would have to differ visually from any sprite in accepted sense of the term.
They can support transparency, they can be flat (ish), they can turn to face the player, they can be animated or not, effectively a sprite.
Additionally - they can indeed hurt the player (on contact) - though making them look like an enemy (character) would only be possible at some distance for obvious reasons, and getting them to shoot at you (or appear to shoot at you) for instance would be somewhat difficult to say the least. It would certainly require some trickery with scripting and alike and I am not really sure that such would be possible at all.
A job for some one who has spare time to try and achieve it.
If you want to see what you can do with decals the best default one to look at I would think is the teleporter one - The floor mounted one. I took that as a basis to study and looked at the way it was set up to create my forcefields which are though much larger being one full wall tile width and height.