Since I do not have access to your contract and your question does not state any of the contract specifics, I can only make some assumptions about your contract and will briefly discuss the statutory, regulatory and DOD policies regarding service contracts.  FAR 37.107 -- Service Contract Labor Standards states the following: 41 U.S.C. Chapter 65, Service Contract Labor Standards, provides for minimum wages and fringe benefits as well as other conditions of work under certain types of service contracts. Whether or not the Service Contract Labor Standards statute applies to a specific service contract will be determined by the definitions and exceptions given in the Service Contract Labor Standards statute, or implementing regulations. 

According to the definition of a Service Employee in 41 U.S.C. §6701(3)(C), it does not include an individual employed in a bona fide executive, administrative, or professional capacity, as those terms are defined in part 541 of title 29, Code of Federal Regulations.

In the DAG, Chapter 10–2.1.2, a service contract is defined as a contract for performance that directly engages the time and effort of a contractor whose primary purpose is to perform an identifiable task rather than to furnish an end item or supply. A service may be either a non-personal or personal contract. It can also cover services performed by either professional or nonprofessional personnel, whether on an individual or organizational basis.

Therefore, I think it's safe to say that  a service contract can cover both professional or nonprofessional employees and can include both SCLS covered and SCLS non-covered employees.  Your question is whether or not a non-covered SCLS labor category can be added to a contract which currently only provides for SCLS covered labor categories.  The bottom line is that it really comes down to a question of scope.  More specifically, would adding the new labor category to the existing contract be considered in or out of scope of the contract? 

In determining whether a change to the contract is in or out of scope, the courts use the following standard:  Would this type of change have been reasonably contemplated by both parties?  To answer this question, the courts will typically look at the following:

1.           The contract requirements

2.           SOW/SOO

3.           Proposal (if incorporated by reference)

4.           Clauses in the contract

5.           Documentation in the file

6.           And finally determine if the original competitive factors have not changed as a result of the contract action in question

Therefore, it is recommended that you review the contract documents for the original contract to try to determine what was the intent when it was originally competed.  If you think the record and the facts support an in-scope determination, the next step would be to request the contracting officer to review any supporting information and to make a scope determination.  Finally, it is highly recommended that you contact your local Contracting Officer and acquisition legal counsel for more information and their interpretation of this issue.