In Magic the Gathering, conceding is a specific action referred to by the game rules defined in a similar way to your other legal actions ennumerated by the rules. If it's your main phase and no spells are on the stack, the legal actions the game has to offer you are playing a land, playing a spell, or conceding, plus a bunch of other things. If your opponent has just played a split-second spell, your only two legal actions would be activating a mana ability or conceding.
So for MtG's Possession, Mindslaver, it requires a specific rule to keep Mindslavering someone and then resigning them from being a legal move. And that rule is there, in the comprehensive rules.
In Dominion, conceding is not a specific action referred to by the game rules, no player is ever given permission by the rules to concede. Being able to concede a game of Dominion is a house rule, really. If you're incorporating that variant rule into Dominion, then indeed Possession should let you win the game that way, unless you make another variant rule to keep people from doing that, which is probably a good additional variant rule as far as variant rulemaking is concerned.
Technically winning a game because you possessed your opponent and made them concede and winning a game because your opponent simply conceded are equally variant and not-dominion. (And all the goko games are not-dominion because you can't see the top card of the discard pile, to be clear.)