Can I add a mesh extender eg EAX20 instead of another MS60 satellite to extend my MK62 system?

Cons: It won't integrate with your system so it might not be as stable. With it joining a mesh system, it might end up connecting to a satellite versus the router and that'd decrease your throughput on a dual band system.

Pro's: its cheaper

I've seen a lot of people just buy another system like the 3 pack and use the satellites while selling the router to recoup some $. I've also just seen then hold onto the router as a backup because if the router fails and you can't get one, you're out a lot of $ in satellites.