The University of Maine System trustees voted unanimously Sunday to support a first-stage $15 million plan to create a new graduate center for business, law and public policy in Portland. The vote, taken at a special meeting in East Millinocket, authorizes the system chancellor to ask for additional funding from the Harold Alfond Foundation, which has been the driving financial force behind the $150 million initiative. Over the past two years, the foundation has provided $2.25 million for early-stage development. If the multi-year first stage is successful, the trustees will be asked to authorize a second stage that includes raising funds to build a $94 million building somewhere in Portland to house the Maine Center for Graduate Professional Studies.