You can supposedly set the stalk resistor values that Hijack recognises:

[hijack]
;; Min/max A/D pairs for all ten buttons on a Stalk
;; Button order is: KOff,KSource,KAtt,KFront,KNext,KPrev,KVolUp,KVolDown,KRear,KBottom
stalk_lhs=0,0,0,0,0,0,0,0,0x40,0x4f,0x53,0x62,0x66,0x7a,0x7c,0x90,0x90,0x90,0xa0,0xa0
stalk_rhs=0x00,0x07,0x10,0x1c,0x24,0x30,0x34,0x40,0x42,0x4e,0x54,0x60,0x68,0x74,0x7e,0x8a,0x94,0xa0,0xa0,0xb5

(I say "supposedly" as I haven't tested it myself.)

You'll need to calculate (edit: or measure) the A/D values based on the resistors your buttons have - relevant info is in the Developer section of RioCar.Org.