Thank you very much @connaisseur@feddit.org
I have tried
bash
apt-get -o Dir::Cache::archives="/to/path" install --download-only apt-offline
But it downloaded only the .deb of apt-offline and not all the dependence tree. Most probably because this machine have them already.
now, remain to force to download also all the dependency tree even if already installed...
Yes, but it's not reliable. because even if you use a bare linux vm to download the packages and dependency, you never know if the online will have already a dependence that the offline system do not have.
no, the only way is to force the dw of the already downloaded package.