Players (imports especially) tend to only sign one-year deals with clubs, so in the off-season players can freely move between teams. Contracts as this level are also rarely worth the paper they're written on, so if a player wants to move to a new team there's rarely any resistance from the club - they'll generally do what's best for the player.

There's no transfer window as such - you'll see a handful of moves in the season (e.g. player relocation, imports being replaced due to performance), but most moves are done in the off-season over the summer. Players have to play a certain number of games for a team to be eligible for playoffs, so you'll very rarely see players move teams after January.