you can, even on steam.

if you bought the game from Stardock directly you can use email or the website, don't remember exactly to register it, then play with auto-login disabled as I do.

steam is a bit harder, you have to go online long enough to install and patch the game, then I would start a game even if it is just to go in for a second, exit, then tell steam to 'go to offline' mode

as long as you do not tell it to connect normally it will let you play. (this works for all games btw)