The Sondors bikes use a rather narrow motor axle (as far as fat bikes go) on the single speed bikes. It's approximately 160mm, whereas most fat bike motors have an axle width of 175mm or 190mm. This means the room to add gears is tight, however not impossible.
If you wish to add gears to your Sondors bike, it's possible to fit 3 speeds with the stock motor. For this setup you would need a new crankset (due to a different chain used for single speed vs multi-speed bikes) a new chain, 3 speed freewheel, derailleur, derailleur hangar adapter, shifter, and associated cables.
It's also possible to fit 7 speeds, but you need a bit more room and it's still a snug fit. To do this, replace the existing 350 or 500 watt motor with a 750 Bafang motor designed for a 175mm dropout. These motors have more room for the gears, allowing for up to 7 speeds. Otherwise the same basic parts are required to make the change.
We do not sell all of the parts to make this swap at Bolton Ebikes currently, but can help with the basic details if needed.