You can modify the code behind on the page located here: ..
\CMSModules\DocumentTypes\Pages\Development\DocumentType_Edit_Header.aspx.cs and add another tab pointing to your module or page:
SetTab(i++, "Test Tab", ResolveUrl("~/CMSModules/Testing/Pages/TestPage.aspx?documenttypeid=" + classObj.ClassID), "SetHelpTopic('helpTopic', 'your_helpdoc');");Keep in mind this file could be modified by upgrades within Kentico so if you make modifications, document them so you know what to do next time if an upgrade overwrites it.