There are two main ways of joining animations. One way is to leave a small gap between them which poser will bridge by interpolation. That can then be tweaked as needed. The other way is to make sure then first frame of the new animation is the same pose as the final frame of the previous. There are a few bvh libraries that use a fixed start and end position to make that easier.

Another option is to make your sections long enough to put together in a video editor (cheap or free) and use cuts or transitions between them. a change in camera position makes that easier visually.

 

I think Poser will load a bvh file starting at the current frame. If that doesn't work you can load the bvh file, and save as a pose file. Then load the pose files.