can i add another electr.heat exchanger to my hot water tank?

i've got a 22mm coil in the hot water cylinder, this gets fed from the central heating boiler and a hot tube ( instead of the electric heater ), this is from the wood fired boiler. i have one 15 mm boss ( low down) left on the tank , but know of no device which would fit, being just 15mm. i would use this in the summer months when neither boiler is on.