A user at the Apad forums says Haipad "official" word (I presume they e-mailed them and asked) was that the tablets also support:
HUAWEI E180, E1553, and E169-U
That would seem to be supported by a list on those forums of modems and providers that have worked by just plugging them in:
HUAWEI E180 Three network
HUAWEI E1750 Three network
HUAWEI E1553 Three network
HUAWEI E1756 Three network
HUAWEI E169-U,E169 Optus prepaid
Also I've seen someone at Slatedroid say the Huawei K3565 is detected, but I don't know if they got it to connect.
For unsupported ones it's not as simple as plugging them in and some may never work, but it appears most Huawei can be made to work: There is a post on another forum by someone who says they actually got a 3G modem working with their M701-R tablet (except they also call it a NuPad). The firmware #133 (09-29) was the first update for this tablet, I'd hope any later release kept the same capability. I'd only update if you're still running the #121. Apparently the issue is that many modems are combination devices that mount as modems and also as flash drives, CD-Images, etc., and Android doesn't know what to mount it as. Also that the modems tend to be Windows-centric so you need a Windows system to "fix" them.
I don't personally know if any of these work because I don't own a 3G modem.