Is it OK to use greenfield, FMC, inside the walls to when installing outlets in old work?
My building is all run in conduit and I want to install an outlet where there isn't one in a inside wall. I was planning on using a metal old work box and wonder if it's code to use metal flex inside the wall from a junction box in the attic? I can't use emt in the walls and run it from the attic because of the roof clearance in that area.

Article 348 allows for the installation of FMC in concealed locations. However, be aware that lengths greater than 6' may require an EGC to be in the conduit.

Also, you mentioned that this is for a building. Did you mean that to be an apartment building? If you are in an apartment building, then a licensed electrician must do this work. No exceptions!