It appears that the process of transferring to Plenti is the same as when transferring to an airline or hotel program. You first LINK your Membership Rewards account to your Plenti account. In this process you choose a name from a pulldown menu. The names will be your own -and- anyone who holds a supplementary card on one of your American Express cards (which earns MR). I doubt you will be able to link to a Plenti account owned by anyone else, but it is simple to order a supplementary card. See this thread for ideas:

http://www.flyertalk.com/forum/ameri...otel-acct.html