Legimet is probably right ("probably" because you have not really presented
your situation) about apt-offline being the solution:
Download apt-offline's package from one of Belenos' mirrors:
http://packages.trisquel.info/belenos/all/apt-offline/download
Bring it to the offline system
Install it with 'dpkg -i' or, if GDebi was installed on your system, by
double clicking on the .deb package.
Here is a documentation on the use of apt-offline:
http://www.debian-administration.org/article/Offline_Package_Management_for_APT
The first package you may want to install with the apt-offline command is
"apt-offline-gui"... so that you do not have to use the command line anymore.
