I actually ran a campaign similar to this when the first Stargate RPG came out (minus the Nazis). I altered the timeline of another universe. essentially the Goa'uld never discovered the stargate on their home world. You could simply say that the Nazis had control of the gate, used it to find advanced technology (you don't really need to explain how), and eventually found P3X-888. It wouldn't be much of a stretch from there to say that Hitler was somehow infected by a symbiote. You could even say the Goa'uld is one from the series, like Ra or Apophis. Although I'm not sure if they would have lived that long. And if you want to have Nazi Jaffa, just say the Nazi scientists are the ones who came up with the technology to implant the larva Goa'uld. Just remember that the Germans will have to discover the gate in Antarctica somehow. Because the gate in Giza was brought there by Ra. And you'll have to ignore the fact that the pyramids where built as landing pads for the Goa'uld ships. As well as ignore that much of human history was influenced by Goa'uld tampering.