How to contribute a converted module? I recently converted a Drupal 6 contributed module to Drupal 7 for one of my client project.  I would like to contribute this converted module to the community.  How to upload this in the existing project page?  Do I have to create a patch or will it be a sandbox project?
For this you need to create a sandbox project and open up an issue in the issue queue of the existing module, posting a link there and urging the maintainer of that project to have a look.

If they wish, they can commit that module as the official 7.x version, or might even add you as a co-maintainer of the module if you are interested. In case they do not find this module interesting, you can create it as a separate project: Create the sandbox, apply for project application approval in the Drupal.org project approval queue, and get this published as independent module.