Hmm I would like to know this too. I'll happily help you with trying to figure it out, in the meantime here is some info that might be helpful.
Generally animations are .fbx exports of a slave rig(basically the original rig has ikhandles, splines, butt-load of contraints, set-driven-keys and what-not, so a slave rig is made to make it easier for the engine to read, which basically parent and scale constrained to the original rig.), which is imported into the engine. So in order to access this you either have to have the original rig, or be able to somehow export them out of your game or trace them in the content list, but seeing as sims work with .package etc. the only way I can think of to access this content is through Sims 4 Studio, or have a very smart programmer who can unzip .package files to reveal the original content. Either way it should be possible to access the rig, because I've seen people make custom "poses" for the gallery pictures taken of the sims they upload, so that's the first clue. Find the rig, find the animations!
