There's a couple of things that might help,

You can use a gcc= cheatcode at boot time to change the version of gcc that knoppix uses, as long as the version is available. You could also try copying the makefile to your usbdrive, hacking it and using make -f .

You might be able to find the the kernel module in binary from, and save it to the usb stick, then insmod it . I'm not sure if that will work or not. Take a look at linmodems.org, if you haven't already.

Generally, software modems are a pain and you will spend a lot of time getting this to work. If you plan on using that particular distro and need to get online alot, look into getting a controller based, usually external serial modem. On the other hand I think Kanotix has the slmodem drivers prebuilt, and you might want to explore using that for a live cd and getting the bioknoppix specific applications to run from your usbstick.

HTH