Install the driver on one kernel, doesn't matter which.
Run the installer using the tarball extract option -x. This creates the tarball directory that contains the source for the kernel interface. You can then compile the source individually for each kernel and copy it into the modules hierarchy manually, either while running the kernel in runlevel 3 or beforehand using target kernel options.
There are excellent instructions available on Zander's site, www.minion.de.
They pertain to the patched installers he has made available, but the process is identical (see his readme on installing the driver into 2.6 with it already working on 2.4).