There are a couple nuclear variants published out there but their rules tend to overcomplicate with resources or new units, trying to emulate the Cold War and straying from Diplomacy's simple design.

here are some rules I hashed out for one:

-All countries start with the same number of ICBMs, maybe 5 total.
-ICBM bases are located at SCs
-Capturing an SC captures an unlaunched ICBM
-ICBMs are not visible on the map, but do appear in the unit orders
-They have two orders: Stand-by or LAUNCH
-If Launch is selected, it can be sent at any territory on the map
-Territories hit by ICBMs disband any units inside it and becomes impassable for one year

That's about it. I'm thinking they can be used mainly as area denial, for big sudden stabs, or setting up potential comeback breakthroughs. The question is do you use up all your missiles as soon as you can or only use them intermittently?

One main issue is there may need to be a one or two year period before they come online, as destroying all of another country's units at the start could lead to some unbalance