[opensuse-buildservice] How to add extra package rpm into OBS project repo but not deleted when project rebuid

I tried to add extra package(rpms) into my OBS project repo(E.g /srv/obs/repos/project/i586), When I run createrepo in OBS server, the repodata will change correctly, and I can use the repo to install this extra package. But when there is check-in to the project, and it begins to rebuild. The repo will republished. I found OBS automatically delete my extra package in i586 directory. And the repodata doesn't include it any more. How can I solve this issue? Hope for your help.