Take a WWI figure, remove the rifle and replace it with a small finishing nail that has the point removed. The rear of the stovepipe will be the "head." Old Glory has infantry that will do.
Take a wwII bazooka gunner and either replace the head with a WWI trooper's head or add a WWI helmet. It is often easier to replace the whole head.
To make helmets out of resin: Get some sculpey mold maker. Form a ball the size of a regular marble and squash it flat. Take an unpainted trooper and use this to make several impressions of the helmet into the mold maker, then bake the "push mold" according to instructions. Be sure to use talcum powder, Bon Ami or something as a mold release. Walmart baby powder is good for this. Contour putty, wood filler or ribbon epoxy can then be used to fill the mold.