Based on how you have described your situation: Yes. I think so.
But please keep in mind that you must earn a minimum of 8 PDUs in technical PM, 8 in Leadership and 8 in strategic and business management. The Agile PrepCast will give you a total of 40.5 PDUs and they are divided as follows: 32.5 technical, 6 leadership, and 3 strategic.
This means that if the 21 PDUs that you already earned include 2 PDUs in leadership, and 5 in strategic then you are good to go. If not, then you will have to earn these specific PDUs separately.
I have 60 hours of PDU to earn for my next year PMP renewal. Meanwhile I am approved to appear for PMI ACP exam. Now bear in mind this means I had already met required 21 hours of agile training. While I am trying to buy prepcast from Agileprepcast to prepare for the exam, my question is really around that 60 hrs... Can I add in my earned 21 hrs from training at work towards that 60 PDUs & remaining 39 hrs would be earned when preparing for PMI ACP exam. Is this doable?