Add HSA contributions after ineligible because I am collecting SSA and retired.

I have had an HSA for a number of years, and it's been super convenient to have funds available to pay prescription costs, medical bills etc. I am now retired and on Medicare so no longer eligible to make tax deductible contributions to the HSA. The account will be exhausted soon so I would like to continue to contribute monthly to use it as a savings account for medical expenses. I know these wouldn't be tax deductible but can I put funds in if I don't deduct them on my taxes? I have two hospital bill accounts I'm paying off monthly on a no interest payment plan and would like to keep those as is using my card.