This functionality - previously removed - has been restored in recent versions of Yast2. Here
is the details, where they state that since version 2.18.0 of the yast2-packager you can enable this feature on a per-repository basis.
For previous versions, until 2.17.?? in OpenSuSE 11.1, you can enable this feature only by using the command line tool zypper
(see the modifyrepo command in the man page of zypper). In alternative, you can manually edit (as root) the repositories configuration files under /etc/zypp/repos.d.
In any case the downloaded packages will be kept under /var/cache/zypp/packages.