Wow.  Here's hoping you have email notification turned on.  And that you still need this answer.

Yes.  It is not only possible, but pretty darn easy.  Well, at least, it is in Crystal XI.  I never tried it in any previous version.

In your bar graph, right-click on any riser (i.e., "bar").  At the bottom of the options list you will see "Trendlines."  This will allow you to create regression lines, and all sorts of other lines.  Honestly, I don't know what half those lines do.  But, I use the heck out of the polynomial lines.

As a note, mine has a weird little bug.  I don't know if it's universal.  But, when you first open the trendlines dialog, and click on a line type, it won't let you select it.  You have to click on a different line type, then back to the first line type.  Then it will work.