I'm trying to find a reference in the FAR as to whether or not I can execute a POP extension on a TO, when the base contract has expired.
Your question and background did not discuss the reason for the extension to the task order, so my answer is written in general terms and probably doesn't take into account the particulars of your specific contract situation. Unless the basic contract specifically places limits on the performance period of the task orders issued under the IDIQ, it should be possible to issue a task order as late as the last day of the ordering period for the contract and the period of performance for the task order could continue past the end date of the contract. However, such an order (as well as any subsequent extensions to that order) must be consistent with the bona fide needs rule of federal appropriations law and the rules in FAR 32.703-3 about contracts that cross fiscal years. Depending on the reason for the extension (e.g. a government caused delay, a change to the work being performed, etc.), determine which clause in the contract is applicable to your situation and read what the clause allows you to provide the contractor in terms of time, cost, or profit. It's also important to ensure the Government receives consideration, if applicable, for the task order extension.
Bottom line: you must follow what's in the contract; as stated in FAR 17.204 (a), "the contract shall specify limits on the purchase of additional supplies or services, or the overall duration of the term of the contract, including any extension." Each task order stands on its own and is basically its own mini-contract. You must read the basic IDIQ contract to determine does it put any limitations on the task order performance period, which clause in the contract applies to your contract situation, does it permit an extension and do I need to get consideration.