Do you want to extend Pega with multiple Solution Frameworks or do you want to extend the frameworks already deployed to your environment? Extending Pega with multiple frameworks is as simple as following the individual deployment instructions bundled with each framework. However, extending frameworks already deployed is a task with "best practice methods" - because when you upgrade to the the latest version of the framework, you will have to manually apply your extensions. For example, this document covers the Insurance Framework.
Yes, it's possible. Some solution frameworks are actually sold in bundles so by definition allow for this. They are tested and there are instructions on how to install/extend them. For ones that aren't bundled, they are likely not tested together in house. That means there is the possibility for problems. Naming collisions being the one that comes to mind off the top of my head. You would need to find and address those issues in your own extension layer, which should be doable. There is more effort on your part, but no technical reason this can't be done. That said, since they aren't specifically designed/tested/validated to work together, there is always the possibility that there is some sort of fundamental conflict between part of framework A and framework B that would require Pega developers to fix. So it should work, but you may have the privilege of being the one to discover the reason I'm wrong.