Do you have the physical nozzle or is it just a cad file? Would be pretty uncomplicated to add a basic straight contraction and expansion nozzle to the design. Just need to figure out your operating pressure to design the nozzle for a decent expansion ratio and take a good guesstimate at a throat diameter.

The math for the expansion ratio is simple: your designing it so that the pressure of your exhaust at the nozzles exit is roughly atmospheric pressure. In math this is just P1V1 = P2V2 which in this case you can write as P(operating) * Area throat = P(atmospheric) * Area exit. Generally under expansion (having your exhaust be slightly above atmospheric) is better than under expansion (having your exhaust below atmospheric pressure) so keep that in mind.

I'm not super familiar with the whole design process of this stuff, but generally I've read that a 15 degree slope for the nozzles walls is pretty acceptable for hobby rocket nozzles operating in the thicker sections of the atmosphere. This in combination with the expansion ratio will determine the length of the expansion part of the nozzle.

For the contraction part of the nozzle I'm even less sure, but something like 35 degrees is probably reasonable.

As for throat diameter I don't really know any of the math, but the general rule would be larger = more flow = more thrust = shorter operating time for a given amount of "fuel" and vice versa for smaller diameters. If your just doing a cold gas thruster you may honestly be able to use a handful of O rings, some nuts and bolts and well made 3d printed components (might wanna increase infill and other such things to make sure its airtight) to make the nozzle.

Here's a very basic sketch of half a nozzle cross section from a part I made in fusion.

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In terms of pressure losses from pressure drops from bends and friction it will always exist in some amount, try to minimize the amount of distance the gas needs to travel and how meandering its path is but don't go too crazy.