Can you learn to sing musical theatre and classical? Which pop singers can sing one of the two?

I was just wondering if you can learn to sing musical theatre and
classical

Also how many pop singers can you name that have either classical or
musical theatre backgrounds?
I know Leona Lewis does, and I think Beyonce and Mariah Carey do. What
about Lady GaGa and Whitney Houston? Celine Dion? Barbra Streisand?
Any others? As many as you can name please!

Thank you! Thank you! Thankyou! Thankyou! Thankyouthankyouthankyou! Oh
btw thank you!!!! :)