or you can fight against vlandia, and eventually a bent, cracked or some other light crossbow will show up with 0 skill requirement. Use the damaged one. It works, is worse than normal, but no other legit way agaprt vland arenas, ending it immediately when xbow isnt a starting weapon.
currently best crossbowmen are mercenary, 180 skill, vlandian sharshooter 130 skil, seemingly gimped imperial sergeant crossbow 80 crossbow skill. 80 skill for a T5 unit is more along the lines of worse than militia, at 90. Veteran militia units like in vlandia, their archers use crossbows, 150 skill.
My current char has some 160 skill in xbow, it was hard work, worth it ? Not really. But to level it up I frequently went alone vs looters, shot them with tournament bolts (worse dmg vs head) and when close, retreated and all over. I shot from 3rd person with the trajectory showing up. In most cases headshots especially from close range are much easier to do from 3rd.
once you have 20 skill, you can either get Crossbow for 1k or very good and cheap arbalest for 2,7k, however, 9k+ hickory xbow is still visibly better.
Currently unavailable best xbow has 70 skill requirement and its damage is just 100. (previously was 119) Considering that maximum amount of bolts is no more than 20 and that bows easily outdamage and RoF is also higher, you arent getting any advantage. Neither in sieges do you have proper big mobile rampart crossbows.