This requires 2 things to accomplish.
1) You need a Mod that allows you to set the next level through scripting. Project Blue can do this.
2) You need a lot of setting up.
I made a freeroam test game a while back when I made the "nextlevel" command. What I did was make a unique script for each "portal" that would lead to a certain map. I would have it set which level was to be loaded as well as a variable that determined where the player would spawn (that's an entirely different subject, but involves placing entities in all the different spawn locations and having a script relocate the player to those entities based upon a variable's value) I put those scripts into triggerzones and placed those triggerzones in front of winzones. The triggerzone would set the next level and the winzone would end the current level.
Hope that was informative enough.
[EDIT]
It wasn't a dumb question. This type of thing is actually pretty hard to accomplish.
The one and only,