Composite ID with property that also contains composite ID

Assume the following class structure:

class Company
public string ClientId public string Code

class Employee
public Company Company public string SSN

The PK for Company is Client ID and Code. The PK for Employee is Company and SSN (therefore Company.Client ID, Company.Code, SSN). However, the fluent interface doesn't support this concept. CompositeIdentityPart assumes the PK of the linked object will be one column. The only change I can see that is consistent is to support multiple invocations to KeyReference whereby if you supply the same expression, it would add a column mapping to the existing KeyManyToOneMapping instead of assuming a new one. Do you agree? I ask because there seem to be a couple different styles for configuring multiple items in Fluent NHibernate. Some use multiple invocations of the same method, some have a method for purposes of a single item plus a collection for multiple items.