Is there a way to transfer a unit to another map?

[steelknight999]

You know like the rexxar campaign where rexxar can enter to multiple maps.

 

[PurgeandFire]

See this tutorial:
http://www.hiveworkshop.com/forums/trigger-gui-editor-tutorials-279/game-caches-6273/

You would use a feature known as gamecaches. Note that this method works in single player, not multiplayer (technically, you can store things, but it isn't preserved after you finish the map).

Just save the hero and all the other necessary information, then you're good to go. In the other map, create a trigger to load it from that gamecache.