I don't think it is possible for Tableau users to create map data layers themselves, but http://urbanmapping.com can probably help. Their technology is behind Tableau's maps, and they offer custom data layer services for Tableau users.
As far as I know, you'll have to submit your data to them, tell them what you need and they will create and give you a file that can be loaded into Tableau which will give you custom data layers.
Dimitri is 100% correct (thank you!). If your data is stored in our platform, we can serve it to you in Tableau. The ability to host custom data layers (amongst many other things!) is part of an enhanced mapping offering that Tableau users can take advantage of.
There's a lot more details at hackyourmap.com about this and other functionality; there you can register for a demo TMS file and explore some sample custom data layers to get an idea of how we can customize the map card.