Trackback Excerpt Question

Maybe this is already implemented in WordPress, and I am completly missing how to do it, but I figured I would post here, maybe someone can point me in the right direction.
I noticed, when doing trackbacks, that if you do not have an excerpt in the excerpt box, that it the trackback will not include an excerpt. Is there anyway to modify that, so that it takes ‘n’ words from the main entry, and uses that as an excerpt?