Considering the Italians did it in 1915 for an 'aircraft MG'. I haven't found any good reason why it wasn't technologically possible beyond doctrine, but even then in 1915 the Germans apparently started development of the MP18, but couldn't field anything until late 1918. Meanwhile things like the WSL rifles were simple blowback and produced by the US in magazine fed, select fire models in 1915-16, so there is no reason either side shouldn't have been able to do an SMG around then. I think the Mauser pistol was adapted for automatic fire early on, but wasn't really an SMG, just a rather unwieldy PDW. I'd imagine a 1914 SMG would be closer to what it was in 1939-40: an officer/NCO weapon for use up close, as they'd be too focused on leading to deal with a full length battle rifle except when things have gotten close enough to require them to start shooting. From here it could evolve into a general weapon much like in WW2.