I am using the plugin MORE FIELDS to define custom meta field, let me explain it by an example, this is the (rss) feed content;
BLA BLA BLA BLA BLA ...
<pubDate>Thu, 15 Dec 2011 03:00:00 +0100</pubDate>
And with the plugin MORE FIELDS I've made a custom meta box so that users when editing a post, if they want, they could enter an url, the key for the meta box is 'webaddres'
In the settings of FeedWordpress, after adding the RSS feed, I went to "SYNDICATION" -> clicked on the name of the RSS feed -> went to Custom Feed Settings (for use in templates) -> added 'webaddress' as a KEY and in VALUE typed the following '$(link)'
So now, if there is a LINK/URL in the rss feed, it automatically fills in the custom meta field of the post and publishes the post with the link/url.
I hope my explanation is clear, English is my third language, so please don't pay attention to grammar/spelling :)