Using PowerPivot with a terribly designed database

I've mentioned in other posts that the primary database I work with lacks primary and foreign keys and this makes things difficult with some software because they detect relationships using them.

TOAD for SQL Server has this problem and apparently PowerPivot does as well.

When I try to bring in 3 tables from this database, PP will not let me link them. In the Table Import Wizard it imports the three tables separately because after I select them (hitting select related tables does nothing) all I can do is select Finish.

PP then imports every record from each of those tables separately. This is how those tables should join:

somast inner join soitem on soitem.fsono = somast.fsono

inner join sorels on soitem.fsono = sorels.fsono and soitem.finumber = sorels.finumber.

The first relationship works fine. It will link somast and soitem on fsono. When I try to llink soitem and sorels on finumber I get the following error:

"the relationship cannot be created because each column contains duplicate values. Select at least one column that contains only unique values"

However, I cannot do so because there isn't one. I realize that I can populate the powerpivot sheet with a single SQL statement beforehand and then drag in all the information, but I'm evaluating this for my users, not for myself.

Any ideas?