Can I join a Medicare Supplement if I already have a Medicare Advantage plan?

No.  It is an illegal insurance practice for an agent to sell you a Medicare Supplement or Medigap policy if the agent knows that you are already enrolled in a Medicare Advantage plan - unless your Medicare Advantage plan coverage ends before the effective date of the Medicare Supplement.

Remember that you will use either a Medicare Advantage plan - or you will use your Original Medicare A and B plus your Medicare Supplement.  But these the two types of Medicare coverage are not the same and do not work together to bring you more benefits.

See the related question:

"Can I join a Medicare Advantage plan if I already have a Medicare Supplement?"
on this site