Okay..I just tested this out for you. No, Custom Scenario does not work, but this approach should work.
Find the Earth18 WB file. The file is actually located in the vanilla Civ IV directory under the folder "Public Maps". Create a copy of this file, so that you don't jack up the original save file.
Open the copied WB file in Notepad. If you scroll down a little bit you will start to see the Civilizations listed. Each Civ has a Team Number (e.g., Team=0). For each of the playable Civs you guys are playing, set this number to the same.
You can now play this in BTS and should see the copied scenario there. Ofc, that approach would be for a new game. For an existing game, you will have to save your current game off as a WB file and edit in NotePad as I described, with the noted shortcomings. Probably best to just start a new game.
Also, not sure what difficulty you folks play on, but I recommend increasing the difficulty. For the Team dynamic, there is some adjustment to tech rates, but ultimately the fact that yall can tag team on research means that you will finish off techs much sooner than normal. You also start with all starting techs of the different Civs in the Team. Plus, you can plan tactical combined attacks that the AI just won't handle and dole out the wonders to build. Another option is to create AI Teams.