Here is how I add new packages missing from stable releases, after getting advice from IRC :

svn cp svn+ssh://svn.mageia.org/svn/packages/cauldron/blobby svn+ssh://svn.mageia.org/svn/packages/updates/1/blobby
svn mkdir svn+ssh://svn.mageia.org/svn/binrepos/updates/1/blobby
mgarepo co 1/blobby
cd blobby
vi SPECS/blobby.spec #update release and add subrel
mgarepo sync -c
mgarepo submit -t 1 --define section=core/updates_testing

=> it fails :

[samuel@localhost blobby]$ mgarepo submit -t 1 --define section=core/updates_testing
Submitting blobby at revision 121955
URL: svn+ssh://svn.mageia.org/svn/packages/updates/1/blobby
error: command failed: ssh pkgsubmit.mageia.org /usr/local/bin/submit_package -t 1 --define sid=35858287-3eca-4f64-bb79-a6153b5061eb --define section=core/updates_testing -r 121955 svn+ssh://svn.mageia.org/svn/packages/updates/1/blobby
A    /var/lib/schedbot/repsys/tmp/tmpgGLj3w/SOURCES-bin
error: command failed: rpmbuild -bs --nodeps --define '_topdir /var/lib/schedbot/repsys/tmp/tmpgGLj3w' --define '_builddir /var/lib/schedbot/repsys/tmp/tmpgGLj3w/BUILD' --define '_rpmdir /var/lib/schedbot/repsys/tmp/tmpgGLj3w/RPMS' --define '_sourcedir /var/lib/schedbot/repsys/tmp/tmpgGLj3w/SOURCES' --define '_specdir /var/lib/schedbot/repsys/tmp/tmpgGLj3w/SPECS' --define '_srcrpmdir /var/lib/schedbot/repsys/tmp/tmpgGLj3w/SRPMS' --define '_patchdir /var/lib/schedbot/repsys/tmp/tmpgGLj3w/SOURCES'  --define 'packager Samuel VERSCHELDE' /var/lib/schedbot/repsys/tmp/tmpgGLj3w/SPECS/blobby.spec --define "distsuffix .mga" --define "mkrel(c:) %{-c: 0.%{-c*}.}%{1}%{?subrel:.%subrel}%{?distsuffix:%distsuffix}%{?!distsuffix:.mga}%{?distro_release:%distro_release}" --define "distribution Mageia" --define "vendor Mageia.Org" --define "_real_vendor mageia" --define "distro_release 1" --define "mgaversion 100"
error: File /var/lib/schedbot/repsys/tmp/tmpgGLj3w/SOURCES/blobby2-linux-0.9b.tar.gz: No such file or directory



If I then modify the spec file (add a blank line for example) and commit, then the submit works.