You can check out Michael Washington's tutorials at http://www.adefwebserver.com/DotNetNukeHELP/ to give you a general idea of how DNN modules work, and some specific scenarios to be aware of.

However, there isn't a basic "how to add an existing user control as a module" tutorial available.  Let me try to explain what I would do in your case.

1. Add the user control to a folder under the DesktopModules folder of your website (e.g. ./DesktopModules/ExistingControl/ExistingControl.ascx).
2. Go to the Module Definitions page under the Host menu
3. In the upper left corner, choose Create Module Definition under the actions drop down
4.  Fill in the information on the form, giving your module whatever name and description you'd like
5.  Once you've clicked update, you can create a module definition at the bottom of the page (again, whatever name you'd like, it won't really matter)
6. Once you've create the definition, you can add a control to the definition.  Make sure that you leave the Key field empty (so that your control will load by default), and select your control from the drop down.
7. Once you've saved this, you should be able to drop your module on a page, and your control will be added.

Let me know if that helps,

Sorry about that, I didn't realize that module controls needed to inherit from PortalModuleBase.  Apparently, all controls in your module will need to inherit from DotNetNuke.Entities.Modules.PortalModuleBase in order to be able to be injected on to the page by DNN.

Hope that helps,