This response is based on the information provided.  We suggest you discuss with your contracting and finance team, program manager and/or legal department as appropriate.

To give a clearer answer, we would need to review the SOW of both the IDIQ SOW and the task order SOW.  Given the question asked, "...a modification to add requirements not on the Task Order SOW...", we are inclined to say NO.  If a requirement to be added is not in the SOW, it should not be added.  If it is part of the task, then the SOW should reflect it.  If the tasks in question are tasks of every task order which have been identified in the IDIQ SOW, then they need not be restated in each task order.

As to the second part of your question:  We do not recommend adding tasks that are not a requirement.  If they later become a requirement, modify the task order at the time they become a requirement.  To list tasks that are not being explicitly contracted for creates potential confusion and could possibly lead to an unintended obligation.  If these are task going to be needed in the future and we know that now, creating options on the task order would be a viable approach.