Can I attach a 3g USB modem to the IAdea players?

No, the player is not a PC. There are no drivers available for the modem to work. You can use a Mifi/cradlepoint router to connect the player to the internet. It translates Wifi to 3G/4G and back. At the time of this writing, IAdea is announcing new players with this capability. Please visit their website for more details.