If you think about it, your blank 20 frames is actually animation. If you think about how the japanese consider music the space between is as important as the notes themselves.
You have 3 animation editor windows in XSI:
0 - the animation editor - for precise editing of motion using curves (eg: use for precise ease-in/ease-out of motion)
Alt+0 - the animation mixer - for re-using/mixing/layering/transitioning and blending animations together
(no hotkey) - the dopesheet - very good for precise positioning/copy/pasting of keyframes
Personally, I'd use the dopesheet (because it is really the easiest way to copy/paste and reposition keyframes for selected obbject). In your case you should:
1.Copy the last keyframe
2.Paste it to the last frame you require.
That way you are properly defining that period where there is no movement for that object. You can tweak the position of the keyframes precisely in the dopesheet by selecting them and using the middle mouse button to move them.
(Copy and paste keyframes using the buttons or holding CTRL+middle clicking and then dragging to a new location.)
Hope that helps.
