Inheritance in LINQ-SQL Framework and/or ADO.NET Entity Framework

I have a problem for which I cannot seem to discern a reasonable solution.
I would like to use the LINQ-SQL classes and objects for doing my database
manipulations but both seem to be lacking in the area that I need and I
would like to know if any of you have a reasonable solution/answer (or just
to tell me it can't be done would be fine).

I currently have a base set of classes setup in a dbml file and the
corresponding code (built using the built-in designer) contained in library
assembly.
I now want to re-use that work that was done and extend those classes in
another assembly.

For example, lets say have I have table Employee in the main database schema
from which I'm deriving this project.
I now have a new database with the exact same schema save that the Employee
table now has an added column that was not previously present in the other
schema.
I want to be able to "inherit" the "Employee" class from the primary
assembly and extend it with an added column.
I have been able to accomplish this feat, and the designer seems to "allow"
this sort of behavior, however, an exception occurs when I try to execute
the code.

The exception reads:

Data member 'System.Guid SetID' of type 'Lib1.Set' is not part of the
mapping for type 'E2Set'. Is the member above the root of an inheritance
hierarchy?

I'm really at a loss here on what to do next or how to overcome this.  Any
help would be most appreciated.

Thanks,
Mike

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