In short, yes, you may make a lump sum out of pocket contribution. The deadline to be counted as a 2020 deduction is April 15, 2021, but you must tell the HSA bank that this is a 2020 contribution and not a 2021 contribution before you make it. And, you must keep to the overall contribution limits for 2020, depending on how many months you were eligible and whether you are covered by a single or family HDHP.
