Taking the PgMP exam without prior certification in project management is possible but has some specific challenges.

Challenge #1 is to get your application accepted (and most likely audited)

Challenge #2 is to acquire the minimum knowledge necessary to take the PgMP in confidence

Challenge #3 is to feel self-confident enough to skip the PMP stage.

All this actually means that even though you do not hold a PM certification, you are familiar with PM best practices and knowledge areas. 

Regarding challenge #2, the good news is that you can only read the interesting parts of the PMBoK and ditch out the unreadable parts. Indeed the PgMP exam relies heavily on logic (tested through situational questions) rather than on knowledge -as opposed to the PMP.

The bad news is that it will take you time and dedication anyway (about 20h) given the sheer size of the PMBoK.

My suggestion is therefore the following:

1- Submit your application for the PgMP certification and see if it goes through the Panel Review (being a PMI member allows you to do that at no cost).

2- Start studying the Standard for Program Management (SPM) which is an easy read compared to the PMBoK.

3- For each SPM chapter, deepen your knowledge of all the topics addressed by refering to the corresponding topics in the PMBoK.

4- Attempt free PMP exam questions (https://www.projectmanagement.com/blog-post/6189/my-top-10-recommended-web-sites-for-free-pmp-exam-sample-questions) until your score reaches about 70% and you feel confident enough in this subject matter.

If you go through these steps & your motivation is intact, then you are good to start the PgMP self study plan.