I recently had an electrician wire my basement that I am finishing. It looks like on one circuit, he installed a regular receptacle outlet (facing a hallway) followed by a GFCI (facing in the bathroom).

I want to add an outlet myself, facing back in to the hallway, after the GFCI outlet. Is this a problem?

Not a problem.
If you don't want the new outlet to be GFI protected, remove the GFI outlet. Connect the new wiring to the existing wiring, black to black, white to white and add a pigtail ( a short piece of wire) to each. Use the pigtail to reconnect the GFI outlet.
OR;
Connect the new wiring to the unprotected line screws of the GFI. The unprotected screws are probably still covered by a piece of tape.

If you want the new outlet to be GFI protected, connect the new wiring to the protected screws of the GFI.

Excellent! Thanks for the help. I hadn't considered it, but actually it would be better if this outlet was not GFI protected... so I think I'll go the pigtail route or the unprotected route.