I installed slackware64-current-mini-install.iso from a few days ago on an Imac i5-3470S after a few false starts with failed full installs. I managed to get it (mini) booting and running and elilo giving me a chooser for an old install (need to keep it for a while) and a kernel 6.9.0 new mini install from about 4 days ago.

Here is the 'issue'. I would like to convert to the full install with the corresponding slackware64-current-install-dvd.iso without booting from the USB stick?

Is there a way to use slackpkg with the mounted USB to install all the packages like the full install but do so using the current minimal install? Or is there some other tool?

The difference between slackware64-current-mini-install.iso and slackware64-current-install-dvd.iso is not minimal install vs. full install. Both isos work the same way: the setup program can do a full install or you can select only a partial install. The only difference between the isos is that slackware64-current-mini-install.iso doesn't contain any packages, so they are fetched from elsewhere during installation. If you gave a net address to fetch the packages and did a full install, it really is a full install.

Regardless of whether you used slackware64-current-mini-install.iso or slackware64-current-install-dvd.iso, if you didn't do a full install, you can use slackpkg to install the rest:

If you mount slackware64-current-install-dvd.iso at, say, /mnt/memory, add line 'file://mnt/memory/' to /etc/slackpkg/mirrors as the only uncommented line. Then 'slackpkg update' and 'slackpkg install slackware64'.