I plan on allowing multiple users to plug their own portable USB modems into an organization's laptop.
Does this mean you allow your users to use a network different from the company network on their laptops and later use the company network again on the same laptop? This would effectively make any network wide security you are (hopefully) using useless, because malware could infect your company network from the external (modem based) network to the company network by using the laptop as a step in the exploitation and thus bypass any network wide protection like firewalls.
Corrupted USB Modem plugged into the laptop infects the laptop?
Possible, but it's less the modem itself but the driver. See also Vulnerability in USB modems allows hacker to access millions of computers remotly or Hacking 4G USB modems and SIM Card via SMS.
Anyway, whenever you give up control about a computer by allowing the user to use different networks, install their own software, use their own drivers etc you must assume afterwards that the system might be compromised and not allow it back into your internal network, in order to protect the rest of the network.