Migrating from one db platform to another is not currently supported. It may be possible to do it with various tools and a lot of time but I have no easy advice for you on that. Once upon a time in a far away galaxy I migrated joeaudette.com from Rainbow portal running on MS SQL into an early version of mojoportal using MySql. I used the SqlYog enterprise, it enabled me to do odbc queries against ms sql and then insert data based on selecting from the odbc queries.
Some day I may make an add on product for exporting and importing data to and from xml but it will not be any time soon.
There is no schema documentation available but the schema is observable using any number of tools.
I would recommend just find a host that supports MS SQL. If you decide to try and migrate, I would first make sure mojopotal using MySql works on that host with a new db or you may do a lot of work and then it still does not work.