I think first we need to work out if wooden/bone crossbow making is different from metal crossbow making. Though ultimately we need to think about combination specialities: i.e. to make a crossbow you need crossbowmaking and bone-/wood-/metalcrafter as appropriate to material. Skills in both appropriate branches go towards expertise in the full task. Either as a job lot or 'work-sharing' (e.g. the existing Architecture then Mason/Carpenter/Metalworker combo for bridges, supports, etc).
Then we can think about whether crossbowmaking is a subset of (generalised) mechanics, maybe change things into a proper skills tree (instead of "those with two or more significantly similarly levelled subskills of a colour-grouping getting a generalist descriptor", a la Ranger for the greens, Farmer for the dark yellows, etc) regardless of how skilled they ultimately are.
If I were wanting to mirror the mechanism in an on-line game I play, then you'd have (say) only the ability to improve general "mechanics" up to what might be equivalent to Novice level, then your choice to work with siege equipment would up your "sieging equipment" all-round skill up to Proficient (but not advance your "pump operation" branch[1] at all, unless you also had that task enabled), and then you could end up with your guy advancing "siege operation" until Professional where (assuming they stuck to the aspect long enough) it'd be "ballista operation", and if I'd planned this example out better I'd have thought of something that a High Master ballista operator could have specialise in in order to gain Legendary status on some extreme sub-speciality there. But such a character would still only be Professional for "catapult operation" (until exercised a bit with that new task) and only Proficient in "siege construction" (ditto) and only Novice on "gear making". Or however you'd have it structured.
But that's just an example, and not a well-thought out one. (Also, I'd be surprised if it weren't already mentioned in the archives. I'm sure I'm repeating, but I'm not going to put my "forum foo" to the test, finding out what form it might have been in.)
I'm also not saying that's how it should develop in DF.
(BTW, while I've gone out and gotten lunch, someone else has almost certainly replied, and I'm betting they've pointed out that normal bows aren't currently Dwarf-friendly, anyway. In a sub-tree version, though, you might have a split for "crossbow" and "longbow" beneath generic "bow", beneath "wood shaping" beneath whatever. And with a multi-disciplinary aspect you'd need the "mini-mechanisms" (up to) High Master-level skill or "cog construction" (up to) Professional-level one or "gear making" (up to) Proficient-level, or rely on your Novice-level general mechanics. And obviously a high "bow" skill (possibly Legendary on the longbow subskill) but merely being at noviciate level in mechanics affects the quality of the item being produced. Possibly having good trading value to one whose ability in whatever subskill of "appraisal" is relevent to aesthetic value, but being below par when actually in use by anyone or when being assessed by an appraiser with a good personal representation in the appropriate mechanics and/or marksdwarf branches of the tree. ("It'd sure look good hung up abvoe the fireplace, but I wouldn't want to have to rely on it when the goblins come raiding...") Alternately, good mechanical sub-trees and low wood (or bone or metal) based skills would make a pug-ugly model that'd be sufficient and more or less practical, just maybe some chance of a structural failure after the first couple of uses.
[1] Arguably also misplaced, currently. If you're looking to regroup [cross]bowyer into the mechanical section, then maybe pump operation is akin to the hauling tasks?