The darcs-fastconvert tool allows you to both import git repositories into darcs (using git fast-export) and export darcs repositories into git (using git fast-import). You may also achieve some success with 3rd-party fast-import/fast-export tools, like bzr-fastimport although this is not explicitly supported or tested. Often, converting from X to git and then to darcs works better than direct X to darcs conversion using 3rd-party tools.