Umm, your bike is quite long. even if you manage to put in a suspension mechanism between the front part and the rear part of the bike, regardless of the length of suspension you choose, the actual amount of vertical suspension will be marginal due to the rather small angle you'll be making with the chain stay.
Also, it's entirely unclear where you would put the bottom pivot for the suspension. Apart from the location near the crank, there's no obvious position where this would be possible. You are looking at a fairly serious frame redesign.