Yes, it's possible to change entities in existing maps. One tool that you can use is BSPEdit
, which lets you edit the 'raw' entity data in a bsp file. Editing that data by hand can be error-prone, so you may find it easier to copy-paste it into an empty .map file and then modify the entities with Hammer or JACK, then copy-paste them from the modified .map file back into the bsp file (.map and .bsp files use the same format for entity data). Oh, and it's also a good idea to make a backup before you're going to modify a bsp file.