Can I add a dimmer switch for lighting to a GFI circuit in Kitchen?
Well, to stay complaint with code you cannot do this. You cannot tap off a kitchen receptacle circuit for lighting. Is it done, yeah, I'm sure it is. Is it correct or code complaint, no, not at all.
You need to find another circuit to take power from.

BTW, the fact that the screws are all used on the receptacles does not mean anything.