No they cannot extend his contract without his consent.
IF his contract expires in 09 the extension gives the Galaxy(MLS) first crack at signing him, but if another team in MLS want Donovan they would have to compensate the Galaxy as in PLayers, money or future players, because the Galaxy hold on to his rights as a player. The contract has nothing to do with other leagues like the Prem or Germany etc. MLS does this as to keep costs down, Donovan is paid 900,000 you can look it up on the Players union website. I have heard that his contract does expire in 2010 not 09. I have talked to some of the Players and Most that are any good will sign a 3,4 year contract with future extensions, but it depends on the player if he wants to stay. I do not think Donovan will resign with the Galaxy unless they change the Rules Allowing a second DP MLS only allows 1 DP per team unless you trade for it there are rumors that there are going to be three. Donovan contract was grandfathered in (not considered a DP) so that Beckham could be signed.
I hope this helps I know it's confusing