You just have to mirror ftp://ftp.minix3.org/pub/minix/packages/ with e.g. http://www.debian.org/mirror/ftpmirror.en.html or http://www.debian.org/mirror/ftpmirror or
wget –mirror ftp://ftp.minix3.org/pub/minix/packages

Size of Minix 3.2 ( ftp://ftp.minix3.org/pub/minix/packages/3.2.0/ ) is about 400 MB (05.07.2011)

next step is to edit /usr/pkg/etc/pkgin/repositories.conf and change the path to your local one.

after a pkgin update you are done 😀