Now that Bazel (http://bazel.io/) has been opensourced, is there an incremental process by which I can gradually migrate (a large repository) from Maven to Bazel?
In the two years since Ulf responded, there's been a few efforts to assist with maven to bazel migration. In particular, the Bazel team is creating a tool to assist with this: https://github.com/bazelbuild/migration-tooling
The tool generates expansive WORKSPACE files from a set of pom files or maven coordinates. In the ideal case, you can pass the path to your maven project, and then it will generate a bzl file you can load into the WORKSPACE file.
More commentary on how to manage external dependencies can be found here: https://bazel.build/versions/master/docs/external.html