For mainstream version control systems such as Subversion, Git, and Mercurial, you may use the Kiln Repository Import tool. You can download the tool from your Kiln account by clicking your avatar at the top right > click Kiln Client and Tools.
You may also access the Kiln Repository Import tool by clicking the Download the Kiln Importer button on any empty repository page.
We do not currently have plans to support more esoteric version control systems such as Visual SourceSafe or Vault. In the case of those systems, you can either create a new Mercurial repository and copy your source over, losing your history, or you can write your own tool to handle the conversion. We are aware that this solution is suboptimal, but it’s unlikely we’ll have a better solution in the future.