From UniMod Wiki

How to add plug-in to com.evelopers.unimod feature

1. Create plug-in using Eclipse wizard
2. Set its version to 0.0.0
3. Open com.evelopers.unimod-feature/feature.xml
4. Go to tab plug-ins and your plug-in using "Add" button
5. Open com.evelopers.unimod.releng/customTargets.xml
6. Go to target with name "replaceVersions"
7. Add tag <eclipse.versionReplacer/> for your plug-in as for com.evelopers.unimod.docs