You should script the tables in MSSQL, then recreate them on mySQL.
next, use MSSQL datatranpsortation wizard to export the table data to flat files and then just import the data into mySQL.
Or, you can write a program in windoze using VBScript + the mySQL ODBC driver program which copies the data from MSSQL to mySQL.
Maybe there is a tool somehint like the MSSQL Enterprise Manager with which you can do the above automagically.
In short, it can be done.
Michiel