The idea is good, but I don't think it can be done because of SQLServer's implementation. The reason you are getting an error after UPDATE is because variables can only be used in the WHERE clause. Thus, you cannot send the table in a parameter as you're trying to do.

>CREATE PROCEDURE usp_updateUserID
>    @Tablename nvarchar(128),
>    @pKey  nvarchar(50)
>AS
>    UPDATE @Tablename  --Got a syntax error on this line!
>        SET UserID= SYSTEM_USER
>        FROM @TableName, Inserted
>        WHERE @pKey.ID = Inserted.ID  --This line I'm not sure about syntax??