Most people seem to reject the idea on principle, but the rules seem to allow it.
"Independent characters are allowed to join other units. They cannot, however, join vehicle squadrons (see the Vehicles section) and units that always consist of a single model (like most vehicles and monstrous creatures)."
The restriction seems to be on units that always consist of a single model, not specifically on monstrous creatures. The parenthetical even says "most". Are there any other rules problems with an IC joining to a unit of carnifexes?
There aren't many ICs in the tyranid codex who would be a worthwhile addition to a squad of carnifexes, but in a team game there are some rather silly combos.