Can I use pin-held V-Brake inserts in (cantilever) screw-held Shimano M Type brake pad holders?
I don't know whether the v-brake replacements fit in. However, normal cantilever brake pads are very easy to find. When you don't find a replacement insert, just order some entire brake pads. You will probably get just as good braking pads for the same price.

Edit: Perhaps ask these guys: http://sheldonbrown.com/harris/brakeshoes.html#dura.