[Alneyan]

Wouldn't drones work to simulate what Brianeyci described? (It has been a long, long while since I Last played Risk, so I do not recall the actual rules of the game) You could build several drones at a single planet, they can warp unlike all other units, and would work almost like ships (once you remove their supply usage).

The two problems are that drones have few movement orders (attack is the only one), and you would need one extra turn to deploy them. The first limitation might be in the spirit of the game though; you would only need to attack one single sector (where the planet is), and not to travel anywhere else in the system. Hmm...

If there is a game master, it may be possible to give extra resources when certain conditions are met, allowing a particular player to build more soldiers where and when she wishes. That requires a game master to be present however; otherwise, I believe you would need some sort of scenario editor to achieve a similar effect.