append-to-register is an interactive compiled Lisp function in `register.el'.

(append-to-register REGISTER START END &optional DELETE-FLAG)

Append region to text in register REGISTER.
With prefix arg, delete as well.
Called from program, takes four args: REGISTER, START, END and DELETE-FLAG.
START and END are buffer positions indicating what to append.

Interactively, reads the register using `register-read-with-preview'.