Don't think it will work as you hope for two reasons: the tankless needs a minimum volume to turn on and I don't think gravity would do it, plus, for a gravity loop to work, there has to be a difference in density of the water. It gets that difference by being hotter than you plan on the storage tank. How is the tank heated? To increase the 'effective' size of the tank, you can run it at higher temperatures. This would necessitate adding a tempering valve, if you don't already have one. If that tank was an indirect and you ran it higher, it may be large enough.