Possible to add lua NPCs/Entities into Hammer Editor? (maybe using lua_run?)

This is for a GMod map, obviously. Title says it all.
(garrysmod.fgd is mounted)

You can add your entities directly by just creating a random predefined entity (say prop_static), open its properties and then directly type in the name of your entity there.

If you want your entities to come up in the list, you’ll have to make your own FGD which you load into hammer: https://developer.valvesoftware.com/wiki/FGD