Hi--
This can be done. You just need to first create a primary key on your table, then set that primary key to be a composite, by using more than one column. At that point, submit will be enabled in InfoPath.
I first tested by creating a composite key without setting a primary key first, and had the same issue of submit being disabled. When I removed that key, set one field as the PK, then edited the PK to use an additional column, I had a composite primary, and InfoPath was fine with that.