Hi Kau,
If your extra parameters are simple and do not require any security, you can just append them to the additionalParameters parameter of the NavigateURL call as follows:
Globals.NavigateURL(PortalSettings.ActiveTab.TabID, "Key", string.Format("mid={0}&myparameter={1}", ModuleId.ToString(), myvalue)
Remember that this value will be displayed in the user's browser and will be directly modifyable, so only use this approach if security isn't an issue.
There are other approaches, but this is by far the quickest.
Hope this helps!
Brandon
EDIT: I'm not on a system with DNN on it, so I can't verify -- the additionalParameters may be a string array, and not a simple string. As such, one may need to modify the code I listed above to reflect the actual parameter type.