Some time ago, there was an application called pkgmanager, available in pkgsrc-wip. It worked by tracking ‘wanted’ packages in pkgsrc, and upgrading based on that list. It hasn’t been updated in some time, however, and may not even build.
‘Rumko’ has written a replacement, called rpkgmanager. The Gitorious page linked in the previous sentence includes the URL to download the code via Git, so it’s available to try now even though it’s not yet in pkgsrc.