You have 2 options:
1. In OST you can use:
./run_suite.sh -r /dev/null $SUITE
when using this option only the packages from the extra sources file will be
taken to the internal repo.
2. When running lago-ost-plugin directly you can use:
lago ovirt reposetup --reposync-yum-config CONFIG --rpm-repo REPOSYNC_DEST --skip-sync
In this case, reposync will not run, but the rpms of the repos listed in CONFIG will be added from REPOSYNC_DEST
to the internal repo.
I'll open a task for automating the second option in OST.