slip in vs add

slip in and add both are verbs.

Word	Noun	Adjective	Verb	Adverb
slip in	No	No	Yes	No
add	No	No	Yes	No

As verbs, add is a hypernym of slip in; that is, add is a word with a broader meaning than slip in:

Other hypernyms of slip in include append, supply.

slip in (verb)	add (verb)
insert casually	make an addition (to); join or combine or unite with others; increase the quality, quantity, size or scope of
	make an addition by combining numbers
	determine the sum of
	state or say further
	bestow a quality on
	constitute an addition