I just tried this. I loaded a figure into the scene, created some keyframed animation for it, then added a new figure. My keyframes for the first figure are still there.

Yes, it's possible to add extra frames into an existing animation. From the Animation Palette, increase the total frames to 240 by clicking on the numeric value and entering the new frame count. Then draw a selection box around frames 151-210 and drag them to the end of the timeline. You'll probably want to make certain that frames 150 and 151 (soon to become 181) are both keyframes before trying this; otherwise, Poser's frame interpolation might produce undesired results in the animation.

I haven't messed around with importing keyframe data from another document, but you could add m frames to the beginning of the importable document so that its animation doesn't start at frame 1.

Yes. First, make frame n+1 a keyframe. Next, draw a selection box around frames 1-n and press the Delete Key Frames button. If you want to leave the remaining keyframes at that spot in the timeline, you're done. Otherwise, select the remaining keyframes and drag them to the beginning of the timeline, then enter the new frame count to remove the blank frames from the end.