I am curious as well... On first look, I would say it's not possible. I don't think the editor has the ability to add teams at a later date.
In the past, the MLS has been hard coded, and I imagine it's still like that. Any changes to the league through the editor would disable all the MLS specific rules (including the draft... my favorite part of the MLS).
I have seen custom league databases that do this for 2016, and make a pyramid style of promotion/relegation between MLS, NASL, and the other lower US leagues. You can check the files here, but I also know that sortitoutsi has a couple that do this.
You may be able to replace an existing team in the MLS (basically, edit all of its information to reflect some other city/team)... but that's probably not what you want.