As usual the VZW rep that replied didn't follow what you were asking.
You can definitely activate the unlocked Samsung Galaxy S20 on your line using your existing SIM card without paying off the remaining device payment agreement balance. You will then pay your next installment on your bill as normal.
If you have questions or your situation is different than I thought, please reply to this discussion thread and clarify.
I'm most definitely NOT a VZW employee. If a post answered your question, please mark it as the answer.