Goal is to migrate some of the builtin objects, but they cannot be selected during a migration session, the Builtin OU is not available for selection (is not visible).
Migration of Builtin objects has been allowed under Domain Pair properties and the corresponding checkbox to skip them has been unselected.
By default you will not see the Builtin OU when selecting objects.
Usually people don't migrate those objects. Microsoft doesn't recommend this, and in order to comply with MS recommendations the tool is not showing any of those objects.
There is usually no business need to migrate those groups, but if this is indeed needed then it can be done.
One option is to create an import file and use this file when selecting objects for a migration, the file will be a simple txt file and will look like:
Another possible option is to start a migration session and select the needed users, then in the same window below press "Select by Group membership" and make your selection, then the tool will automatically select all those groups and you will see them in the next windows as selected. And this will also take care of group membership population.
Note: during the migration one should specify to leave the accounts where they are. The tool will migrate and merge/match them. If you include the users (members of those groups) in the same migration session then the tool will take care of the group membership and will populate the groups with members. Otherwise you will need to rely on the dirsync.
SOL16917 has more information about the usage of an import/selection file.