My essential tips for anyone new to Poser animation;

1. Learn to use the animation palette and graph editor, underlined in capitals. Unless you understand these, you'll be fighting phantoms in the fog.

2. Don't confuse your .pz3 with your movie. Think in terms of creating a sequence of film clips to be assembled in a video editor. ( my favorite free one, VirtualDub)

To create a continuation scene, save your file, then save again under a new name, in the animation palette go to the last frame, make everything a keyframe, select all the preceeding frames, delete them, then drag all the last frame keys back to frame 1, continue your animation and render from frame 2, you already have frame1 rendered as the last frame of the previous segment.

Lastly, a quick tip for the perennial slidey feet issue, don't look at them! Move the camera closer or tip it up a few degrees...."b, but, that's cheating!"... yep, in time you will hopefully learn how to correct such issues, animating in Poser, especially when starting out can often be a compromise between what you want and what you can achieve. Creative solutions to problems will be necessary. Posting them here might help us all.