I'm not sure if I fully understand what your problem is, but the following approach (exact details could be wrong) should work.
You can create a local repository with dpkf-scanpackages and then add an entry to your sources.list
dpkg-scanpackages /mnt/storage/apt/ | gzip -c > /mnt/storage/apt/Packages.gz
Add to your sources.list:
echo "deb file://mnt storage apt" >> /etc/apt/sources.list
And don't forget to run 'aptitude update'.