Model definitions

I've inherited a database from a vendor. This database has primary keys defined but not any foreign keys to other tables. The vendor establishes all their relationships in their code.

I've created a simple model off of the base tables, but can only report on one table at a time which makes perfect sense. What is the best way to create a more usable model

Create views with foreign keys defined and then use the views as my source for the dsv or can I define relationships within the model itself I have not been able to figure out how to add a foreign key in the model Can this be done