Edit a silkscreen over a gerber file in Altium
If I understand your question, that you want to edit the silk screen gerber without changing the original pcb file, yes, it's possible. 

You use Altium's included CAM editor (CAMtastic). Stack Exchange won't let me link the manual section because it's a numeric URL, but you can find it easily. 

Of course next time you generate the output job (because of a revision, say), you'll have to repeat the editing, so it's usually not the best way to do things.