Different Systems of Investiture are accessed differently. Returned are Spiritual Magic users, and feed on Spiritual Investiture, so they can subsist of Breath, Stormlight, or possibly the Dor. Allomancy is Physical Magic. It's power by the Physical Fuel of the Metals. It can't be powered by anything else, other than the Pure power of Ruin of Preservation. An Allomancer that sucked in Stormlight somehow, would gain the Passive abilities of a Surgebinder, but nothing else.
South Scadrilian Mist Fabrials though, you might get away with powering them with stormlight, but I'm not certain how that would work.