Assuming you are talking about a JAA license, as far as I know, if one holds a JAA CPL and wants to get the JAA ATPL the person must take the complete ATP theoretical exam(s). I'm not sure about the check-ride (skills test), if the person already holds a valid JAA CPL+IR it might be waived. I suggest to contact your national aviation authority and get that cleared with them.
Good luck
PS. the following link might be of any use for you:
Commercial Flight & Aviation Training - Scandinavian Aviation Academy