You mean like a read more link?
I would just use the FWP Limit Post Size Plugin.
Charles has so many extras for feedwordpress, he's truly fantastic.
http://feedwordpress.radgeek.com/wiki/add-ons-and-filters
I use Limit Post Size. I used to use the excerpt reloaded but moved to this instead and haven't had an issue. (It saves me some time rather than fuss with the template.)