Airprint on older HP Printer (Color LaserJet 2600N)
I don't know that you'll be able to install firmware into the printer that  lets you accept jobs directly from iOS. However, if you have a Mac running on your network, you could try something like AirPrint, which runs on your Mac and makes all the printers available on the Mac available to Airprint as well.

This works great for me with my old Mac Mini that I run as a server and an HP CP1515 printer. This might not be an option if you don't have a machine available all the time, but it's a great solution if you do.