Assuming your application is running in rails already, you can run
'rake db_schema_dump'. This will create db/schema.rb, which you can
then modify before importing into your MySQL database by running 'rake
db:migrate'. If you're not already running Ruby on Rails, you could
always set up a new rails application and point it to your current
MSSQL database (instructions here:
http://wiki.rubyonrails.org/rails/pages/HowtoConnectToMicrosoftSQLServer).
You might be able to use ActiveWarehouse ETL to extract from the old
system, transform the data as necessary and load into the new system -
that's what it was designed for (although primarily for data
warehouses but there is nothing that locks it into that).