First off, check to see if there are any PPDs from HP. Generally, if HP has some for Snow Leopard you can extract the PPDs from the install package and place them in the proper printer location.
Then, just add an IP printer. Since this is an HP, I'd try the HP Direct Jet protocol first. Give the address as the IP address of the Airport Express Base and use the HP PPD you just extracted into the correct printer folder for the printer type.
If that does not work than try the IPP protocol.
If there are no PPDs at all then you might try looking at the Gutenprint project or finding an HP printer that has similar enough PPDs.