I think you will get some heat from the coil mounted externally behind the stove, but if the water is circulating I doubt if you will ever create steam in the coil if the only heat source is the stove. It is pretty hard to create steam in a coil even when the coil is mounted inside the stove. [providing water is circulating]. If I remember correctly from posts on the forum, only after a night of high stove temps.
You would be keeping the water in the steam boiler warmer than without the coil, but it will depend on many variables whether it will be enough heat to keep the water hot enough to prevent the oil burner from running.
The big question is what happens when the water in the oil boiler is heated to create steam? If the water is under pressure, and just below boiling temperature, it could possibly flash to steam in the coil, and what this would do to the water trapped between the coil and the return to the oil boiler is anyone's guess.
I'm inclined to recommend putting in a tempering tank prior to the hot water coil in the steam boiler, and let the coil in the stove preheat this water. It will greatly reduce the oil heat needed to heat the preheated water to a comfortable domestic water temperature. This would keep the steam system and water system separate.
I'd be cautious mixing the steam system with a water coil, hot water is dangerous enough, but steam can develope some high temps and pressures.
Hopefully someone with experience in steam systems will offer their expertice.
Hope this helps, Caution and safety first. Recovering from burns, even minor ones is not something to risk.