If you have a dependency on a third party artifact that is not in Maven, you should:

1. Ask the project to add it if at all possible. Most open source projects want wider adoption, so this kind of request is often well received.
2. If they won't add it, we may be able to add it to our Maven repo, assuming it can be published at the ASF at all (no GPL code, for instance). Please ask on the mailing list first.
3. Assuming it can be, then you need to sign and deploy the artifacts, as described below:
   mvn gpg:sign-and-deploy-file -Durl=https://repository.apache.org/service/local/staging/deploy/maven2 -DrepositoryId=apache.releases.https -DgroupId=org.apache.mahout.foobar -DartifactId=foobar -Dversion=x.y -Dpackaging=jar -Dfile=foobar-x.y.jar
4. Once it is deployed, go into http://repository.apache.org/ (use your SVN credentials to login in)
5. Select Staging
6. Find your repository artifacts
7. Close them (this makes them publicly available, since you are closing the staging repo)
8. Promote them. This adds them to the public Maven repo.