I'm new to Middlegen and wanted to use it to create hbm.xml files out of a quite big legacy database.

I have got relations without keys defined in it and Middlegen generates composite-id's out of all attributes when no key is defined.

Is there any way to force Middlegen to create specified keys without to change the database schema?

When it finds there is no primary key(s) defined it will generate a single large composite key since it has the best chance of being unique. The Relational model your using is not correct (which I sure you already know). It is too difficult (where the reality is i'm not motivated) to allow custom selection of fields that can pretend to be the primary key. I suggest you hand edit the table HBMs in question or better still add the primary constraints to the legacy database. Maybe you would be allowed to just change the development database to help.