you can't assign it with q-link. You do it in either the midi or the "other" mode menu. I use jj0s2 though i believe this is do-able in jj1 if thats what you're using. I just got a stupid-cheap sustain pedal (just a little plastic boxy deal) at best buy for like 8 bucks. there's a menu in one of those modes that'll let you assign it to a particular pad or to transport functions. personally, i use it for recording sample duration (another option in midi) as the action on these types of pedals doesn't really lend itself to drumming in my opinion.
edit: something like this will do. I'm not sure what the polarity needs to be with the mpc (or if it even matters) so unless someone else can chime in I'd just get one that allows you to switch it like the one here.
http://www.samash.com/webapp/wcs/stores ... -MPS25XXXX
