I think I found a simple way of saving a copy of any installation package (whether coming from Nokia Store or manual apt-get command), at installation time.
It is a one line hack, so I'm not going to wrap it into a user-installable package.

So, open a terminal session as root, go to /etc/apt/apt-conf.d/, launch your favorite editor and create a file named "02backup" containing the following:
That's all.
Since the next installation, a directory /home/user/MyDocs/AptBackup/ should be created and all packages should be copied there.
I haven't tested with packages that download dependencies, but I believe also said dependencies will be 'backupped'.
Note: you may want to disable this feature if you are going to upgrade to a newer PR release, otherwise your upgrade will be slow and your storage getting filled.