Absolutely. If your consciousness is present, you can learn things about your consciousness. How to access different states of consciousness, realizations about the nature of consciousness, whatever can be learned through meditation.

And, aside from that, honestly, I come to a lot of my knowledge/realizations about particularly STEM kind of subjects long after having had the external stimuli that is educational study. I didn’t intuitively “get” logarithms until long after I finished college, and I don’t think for any reason related to any external stimulus at the time. I was certain my realizations were “correct” without external verification. I had everything in my mind already necessary to confirm them. And when using that knowledge in situations that did involve external stimulus, the realizations bore fruit. (To put the realization into words, it would probably be something like “logarithms are roughly just a measure of how many digits an operand is in a given numerical base.”) I’ve had similar realizations long after the fact about trigonometry.

There’s also the possibility of recalling things you’d forgotten.

Several people here have raised the objection that without external confirmation, it can’t be called “knowledge” as there’s no way of confirming it. But I’d counter that there’s not really a difference. There are ways to confirm knowledge without external verification (“thought experiments”, for instance). And there are limits on what can be verified and what can’t be verified even with interaction with an “objective external world.” (Even with an external world, how can you be sure it’s external and not something you’re making up as you go – a believe called “solipsism”? Short answer, you can’t. So can you claim as “knowledge” anything you “confirmed” by interaction with the world you think is “external”? How can you be certain you’re sane enough to be able to trust your confirmations? You can’t, and the fact that you can’t doesn’t hinge on whether you have access to an external world.)

Yes, there are limits to what can be learned from the external world. (For instance, you can’t verify General Relativity is something that’s a thing in “the external physical world” (assuming there’s only one, that is!) without experiments in the external physical world which you hypothesize may be well described by General Relativity. But if you came up with General Relativity on your own without external stimulus, you could learn many of its consequences should it prove true. And “if this then that” conclusions can definitely qualify as “knowledge” even if you don’t know if the “this” is true or not, I’d say. ) There are also limits to what can be learned from interacting with the external world. (Like realizations about your own psyche.) I think you’d have to pretty much ignore the “hard problem of consciousness” entirely just because it’s inconvenient to conclude that you couldn’t learn things without external stimulus.