Sort of! We don't have any particular default options for just instantly making your league into a "keeper", because everyone wants to run their keeper leagues differently. However, we do offer a ton of tools for helping you administer them. I think they're described in some amount of detail here: http://help.yahoo.com/l/us/yahoo/sports/fa...seball-130.html .
If you're specifically asking about keeper league support in the webservices, we don't have too much there right now (as keeper leagues are in the vast minority of leagues that we have out there).