To my knowledge we can't use creatures for reactions like the milking/shearing jobs do. We also can't specify which units get used. It just marks the nearest eligible unit for the job.
Actually you can if they are egibile in the actual creature raw and line up ok with your reaction and material code.
For example leatherwing bats in modifications (bats that grow excess 'leather') can be sheared for the defined product by use of local creature mat, which trolls share with sheep and alpacas in "yarn", which is one of the legal
material usage tokens(second half of the hyperlink) defined to be put into these kind of reactions to which Toady has concealed from us so we dont precisely know how it works. Because the stockpiles are prefixed, it has to be this way (not everything there will work however so be selective and concise)
[BODY_DETAIL_PLAN:STANDARD_MATERIALS]
[USE_MATERIAL_TEMPLATE:HOOF:HOOF_TEMPLATE]
[USE_MATERIAL_TEMPLATE:HORN:HORN_TEMPLATE]
[SELECT_MATERIAL:HAIR]
[STATE_NAME:ALL_SOLID:wool]
[STATE_ADJ:ALL_SOLID:wool]
[YARN]
Same for milking which is defined by [milkable] (which puts a timer on the fluid dispense and into a bucket, but can theoretically be any said material)
[USE_MATERIAL_TEMPLATE:MILK:MILK_TEMPLATE]
[STATE_NAME:ALL_SOLID:frozen reindeer's milk]
[STATE_ADJ:ALL_SOLID:frozen reindeer's milk]
[STATE_NAME:LIQUID:reindeer's milk]
[STATE_ADJ:LIQUID:reindeer's milk]
[STATE_NAME:GAS:boiling reindeer's milk]
[STATE_ADJ:GAS:boiling reindeer's milk]
[PREFIX:NONE]
[MILKABLE:LOCAL_CREATURE_MAT:MILK:20000]
Because orders to shear and milk overlap, you will have to divide your animals into burrows so that they aren't constantly getting muddled up and that your shearers and milkers remain on task with the set animals in that range. Additionally make sure to put sustainability commodities for both your dwarves and your animals to keep them alive as nessecary when they are positioned.