Can I create a CLIN 4003 on the Task Order for Labor without modifying the based IDIQ contract?
The FAR doesn't prohibit creating a new CLIN at the task order level without including it in the base IDIQ contract. But, I think it's fair to say that doing so is not the norm. I think one reason is the challenge it would create with tracking CLINS and funding at the total contract level. For example a stand-alone CLIN on this task order and a stand-alone CLIN on that task order (which are not tracked at the base contract level) will most likely cause some continuity issues in terms of keeping track of the CLINS and funding on your contract. But, even more concerning is the question it raises about scope. Adding a new CLIN usually implies new work is being added and would not be in scope of the contract. In these types of situations, the contracting officer has a duty and responsibility to complete a scope determination and determine if a Justification and Approval (J&A) needs to be completed in accordance with FAR Part 6.
You also mentioned that your team had a discussion about whether a task order is "stand alone" from the base IDIQ contract. That's a question that generates a great deal of debate in many program office circles, and the FAR doesn't provide very much help in answering that question. However, I think many people would agree that the task order should be issued under the terms and conditions that are contained in the mother contract (See FAR 4.1005-1 and FAR Clause 52.216-22(d)). Also, the work to be performed in the task order is normally tied to a "corresponding" line or subline item from the base contract (See FAR 16.505(7)(iii)). Therefore, someone can try to say that the task order "stands on its own" from the base contract (and there's room for interpretation about what that really means), you still can't get around these two realities: the T&Cs in the base contract are what govern at task order level, and the task order must include the corresponding CLIN from the base contract (unless the work is out of scope).
Bottom line: Adding a new CLIN to a task order but not the base contract is not prohibited by the FAR but can cause problems as outlined above. Whether or not someone wants to make a case that a task order (or a new CLIN) is "stand alone" kind of misses the point in my mind. The more important question to ask is if it's in scope of the contract. I think adding a CLIN to a task order, which is not in the base contact, raises some legitimate concerns about whether or not the new CLIN is "in scope". Finally, any decision regarding the CLINs on the contract need to also take into consideration your Service policy, agency policy letters, and ultimately your Contracting Officer, and we recommend consulting with your legal office to ensure any legal/regulatory implications are fully understood prior to taking action.