The itc's are defined in header_items.py, but the big red letters at the top say not to edit that file, so I'm guessing we're not to edit it

I'm not sure whether defining new itc's at the top of module_items would work, though; the imodbits constants are defined that way and it works; anyway there's no harm in trying it, just enter a new definition, use the itc, and see if it's recognized when you try to compile.
Regardless, that's not really necessary though, as you can assign any combination of capabilities to an item. Just separate them by vertical lines and make sure you prefix "itc_" to combinations and "itcf_" to individual capabilities. Here's an example from the 1066 items file:
插入代码块:
["mallet","Mallet",[("mallet",0)],itp_merchandise|itp_type_two_handed_wpn|itp_primary|itp_two_handed,itcf_overswing_twohanded|itcf_slashleft_twohanded|itcf_slashright_twohanded|itc_parry_two_handed,29,weight(3.8)|difficulty(8)|spd_rtng(55)|abundance(8)|weapon_length(105)|swing_damage(35,blunt),imodbit_cracked|imodbit_bent|imodbit_balanced|imodbit_heavy],