I've not ran the netinstall for a while but I believe there is a way to add custom kernels.
You might just be able to extract this package (if it matches the installers kernel) as a tarball and copy the module manually into /lib/modules/2.6.26-2/kernel/drivers/fs and do a modprobe reiserfs. That is a bit hackish though, but is all I can think of because debian is so damn reliable any system I put it on never ever needs a reinstall.
While I am here I might point out that I think sqeeze isn't unstable enough to warrant using lenny and its outdated packages. I've been using squeeze for a while now with no major problems. It soon will be the stable release.
If you DO use lenny you will want to use
www.backports.org to get the latest firefox, openoffice, and so on.
