To add a CUPS printer, use the 'hp-makeuri' command.
e.g. for a network printer:
$ hp-makeuri 192.168.10.100
<...>
CUPS URI: hp:/net/HP_LaserJet_5100_Series?ip=192.168.10.100

Then in the CUPS interface, add an 'HP Printer (HPLIP)' entry with the
corresponding URI.