"I'll Pencil" (because the appointment can be erased) "you in" (to my schedule book).
Boohiss: Er. I'm kinda busy this week. Tell you what, I'll pencil you in for 4:30 on Thursday.
(Pencil, a noun, is being used as a verb for the action associated with it.)
Daphne: I'm not free until 6.30. I'll pencil you in for then if that's OK.
Fred: It's a date.