Does this mean I need to generate a full file for each model for each animation sequence?
Each file will contain the textures, mesh and bones with one animation sequence?
Then I can load one, and then append the others to add the additional animation sequences?
Yes, if you want to have separate animations (using the same mesh, bones etc).
Is there a way to write a b3d file that is only an additional animation?
No.
Don't get confused, read the flowery manual! :) Blitz3D has several ways of doing animation stuff, I'd use the "all animations in one file" and Exctract commands, but I just wanted to show that it's allright to have several separate animations too and compile them together with LoadAnimSeq. It's fully up to you to decide which method is the best choice for your animation pipeline.