Custom matches have been possible since 3.4, though it's unranked, and there are no rewards given the nature of it. It's a great way to practice, and you can even chat normally during it. The system works similar to forming an alliance (team 1 + team 2, with an optional spectator team, party leads invite to custom match).

As of 4.1, cross server custom matches are possible as well, so if you have friends on other servers, it's entirely possible to arrange matches with each other. Again, these are unranked, and offer no rewards, but they are a great way to practice, or just have some friendly matches with people you know.