The only options are the one mentioned or burn the iso to disc and use the install game option in HDloader to install the game from the PS2's DVD drive. However that method is very slow and you will need a blank DVD-R for every game you want to put on the HDD.
HDLGameInstaller (on console) + original disc (on console)
HDLGameInstaller (on console) + chip + copied disc (on console)
HDLGameInstaller (on console) + Windows utility (over the network) - best reliability, IMO
HDL Dump Server (standalone homebrew, or built into OPL) + hdl_dump or hdl_dumb (over the network) - capable of advanced operations such as making PS1/homebrew HDDOSD bootable partitions
hdl_dump or hdl_dumb (with disk connected directly to PC) - potentially best performance
WinHIIP is not recommended anymore (for installing games, at least) due to the above, more updated replacements (which do not attempt game-specific hacks that sometimes it gets wrong)