... for making it possible to bring withering properties into gear and potion!
edit: or, in an easier way, make the withered roots able to give potions and gear this kind of property
Though interesting, I expect the IC implications of a PC being able to inflict and directly control the Withering may be a bit too dire for this to become generically available.
You could always attempt to earn such things through efforts that aid H'Bala and her agenda. :)
Best handled IG. It might already be possible for Bestow Curse to also increase withering if you are a recognized agent of H'bala, or another means.
You could always get with a DM and RP it? Maybe dip some withered roots into a potion cache and hand 'em out or something?
I'd support this if it came with the same sort of EXP drain that removing the withering came with.