You'll be able to use the general sign up form. If you want to participate in the exchange and are also willing to pinch hit, there is a check box for that. If you don't want to participate in the exchange but want to be available for pinch hitting, you can put "n/a" in the requests sections and then fill out the part about what canons you're willing to write for, etc., and mark that you can be contacted for pinch hitting.