Can I send IRS a payment for a prior year that is not yet filed?

Hello everyone. I am having a hard time finding an answer through search, so hopefully someone here can help.

Is there a way to send a payment to IRS for a prior year as yet unfiled? I would like to not delay any further in getting a payment in while I am working on the filing. Some specifics: My wife and I ultimately file a joint return by her amending her timely filed return to include me. In most cases, my coming in on her return results in a nice refund because she essentially overpays by initially filing separate. It seems that will not be the case, however, in the year in question. It seems likely there will be more tax due for that year when we file the amended return. I have filed an extension form in the past for the current tax year with a check attached, and then had the check amount credited to her (our) amended return. The problem I'm having is that I am not finding a way to do this for a prior tax year. I realize there IS a difference in that it's not an extension, but I'm thinking IRS would be delighted to take my money for a prior year while we are in the process of preparing that amended return.

Any knowledge shared would be appreciated! Many thanks.