Okay, I've got the animation offsetting correctly. The link stwert provided works perfectly - my earlier error was due to corrupt archives that weren't importing properly.

So I created a map in the "frame" field of the xgen description, then added an expression. It looks like this:

$a=map('${DESC}/paintmaps/TestFrame');#3dpaint,200.0
$a=20*$a+$frame+1;
$a

This offsets the animation properly. But what if I want the animation to loop - that is, when I get to frame 420 (which is the final frame of animation imported with the archive object) I want the expression to start over at frame 1. As currently written, when it gets to the value 420, it just stops animating.