If you have create .Net class that represents some user-defined type, compiled it into .dll, and imported it into SQL server as an assembly, you can create user-defined function that references this class:
CREATE TYPE dbo.Point EXTERNAL NAME MyLibrary.[Name.Space.Point]
You need to specify name of the type that will be used in T-SQL queries. In EXTERNAL NAME clause you need to specify assembly name, namespace, and class name.