United has entered into negotiations with Driver, who has expressed interest in remaining in MLS.
Driver has a green card and does not count as an international player. With only one international on the roster, Canadian midfielder Kyle Porter, United doesn’t have to protect him ahead of the expansion draft Wednesday for Orlando City and New York City FC. Teams with multiple foreign players must protect at least one.
Willis, 26, is out of contract this month and has been exploring European options. He was with United for four seasons and started 19 league matches between 2012 and ’13 but did not make any league appearances this season after slipping to No. 3 on the depth chart behind Bill Hamid and Andrew Dykstra. He served much of the year with third-division Richmond and was recalled when Dykstra tore an Achilles’ tendon in late July.
Dykstra is expected to be back to near full strength when training camp opens in late January.
The Dynamo was in need of goalkeeping depth after trading starter Tally Hall to Orlando.
Imkoom, 25, barely had time to unpack his bags in Washington. A 2014 Ghana World Cup member acquired by United in September, he made two league appearances, one start, and proved his value in the CONCACAF Champions League by assisting on two goals in three matches.
United also acquired allocation funds from both Orlando City and New York City for international roster slots the next two seasons.