Different solutions :
- use the last OPL version (0.9.3) - as it has now a cheat engine. Only RAW codes are accepted. I can profive more infos if you're interested.
- patch your iso with RAW codes again using PS2 Patch engine :
http://psx-scene.com/forums/f19/ps2-patch-engine-117652/
(you need to convert your ARMAX codes to RAW before using it).
But no, AFAIK, it's not possible to do it directly with ARMAX codes - since they are encrypted.
Side note : HDLoader is old & obsolete now... better use OPL 0.9.3 :
https://bitbucket.org/ifcaro/open-ps2-loader/downloads