The easy method is to copy the content of /var/cache/pkg/ to USB device.
Another option is a portable mirror
Techinically you would do something like
- create a list of repo packages installed
- ensure all packages is present
sudo pacman -Sw - < pkglist.txt - prepare some adequate storage
- copy the packages to the storage
- on the new system - feed the package list to pacman
sudo pacman -Syu --cachedir <your-storage> - < pkglist.txt