I'd like to combine the words "apis" and "genu" into an English word that means something like "pertaining to the bee's knees". My guess would be something like "apigenual", but I have no idea if that's close to the mark or not.
You're actually right on the money. Note that genualia (from a hypothetical *genualis) was used by Ovid to mean "garter," but it's a hapax legomenon, and thus Ovid probably coined it with this meaning himself. This sort of combining words was unusual (outside Lucilius' ridiculed efforts), but here it's pretty clear what the word would mean.
The breakdown of the word would be a series of questions: what it's pertain to (-al)? Knees (genu-). What kind of knees? Bee's knees (api-).