You should be able to do this with apt-cdrom. Just put and entry in your /etc/fstab and then pass the correct argument to the -d option.
Eg, something like:
echo "/dev/sdb1 /media/usb auto rw,user,noauto 0 0" >> /etc/fstab
apt-cdrom -d /media/usb add
apt-get install whatever
Where I'm assuming the the usb stick will appear at /dev/sdb1.
But this all prompts the question: why can't you just use apt-get as usual and install packages from the repositories on the network?
PS. There should be no need to reinstall.