theoretical it´s possible - but i will be a not a quick job. You can´t have more than one kf for one animation (you only can have different version (therefor the 10xx, 20xx and 30xx numbers in the kfm), but only one will be played).
You can manual add new controlled nodes to a kf in nifskope and copy the one you like, but you have to match the time (there are certain sequences with fixed times, so there will be no real problem, but as example run and idle can have different length) and check the name (the affected bones in the nif is controlled by the name - case sensitive)
to your actual problem - the horse animations are not really suitable for the camel, the rigging between both units differs greatly and you will have without alot nifwork alot deformation. But what you could try is to exchange the upper body parts of the rides with an animation of the unit you want to create (eg you want a camel musketeer, you could try to use the camel arches as base and add for the upper body the animation information of the cuirassier).
[not that i expect there will be no problems doing this, different time of kfs , eg]