We have an older DOS application (Wintek) which can only print out to
Epson FX/MX, IBM Pro and other DOS-generation printers. We can also
create a disk file of the output.

How or where can we support DOS printing or at least the output file
conversion into modern Windows printers such as HP inkjet printers?

There are applications that you can use to make a new printer print through dos

such as: dosprinter 11.00

The other option, is figgure out what format it saves your file in dos
in, if its plain text you can read it with notepad or word.