The concept of a PDU (Professional Development Unit) only exists after you become a PMP (or PgMP). It means that you are working on developing yourself even after becoming a PMP. PMI know that you have been working on your project management skills before becoming certified (and that's why you got the certification) but they want to make sure that you keep on enhancing your project management skills and updating yourself with the latest in the project management world after getting certified.
In conclusion, any project management activity that you want to claim for PDUs has to happen after you becoming certified.
There is an exception to the above, however. Let's take the following example. You are PMP certified for 2 years, and then you apply for a PgMP and pass. Your renewal date for the PMP is in 1 year, and you've already accrued the necessary PDUs (60). Now if you align the renewals of both the PMP and the PgMP certifications you will have to renew both the PMP and the PgMP in one year, and in this case, the PDUs you have accrued in the previous 2 years and the next year can be used to maintain your PgMP as well. I believe that very few people fall under this situation, but I thought about mentioning it anyway.