It is possible to MSSQL Provider for Joomla? I understand that there's a MSSQL Client for PHP in beta and that i question whether it might be difficult to alter the database provider to suggest to MSSQL rather than MySQL.

Based on this thread (particularly this publish) within the official Joomla forum some queries within the Joomla source code are MySQL specific and don't focus on Microsoft SQL Server.

It's Joomla itself, some queries within the code are specific for Mysql. Making Joomla work with MSSQL right now includes searching through all queries, checking which of them are Mysql specific and alter these to MSSQL or generic ANSI SQL specific.