This is a request we've received a couple times now... to essentially improve the search features within the Add Relationship dialog. It is in our product backlog of features.
Unfortunately I can't say when it might get changed, but we do appreciate the feedback.
the Telerik team
Registration for Q1 2011 What’s New Webinar Week is now open. Mark your calendar for the week starting March 21st and book your seat
for a walk through all the exciting stuff we ship with the new release!