So here we go, but in order to complete this guide, you need to have a software to decompile Sitecore.EmailCampaign.dll.
- Firstly, try to find this file \Website\sitecore modules\Shell\EmailCampaign\UI\Dialogs\ImportUsersWizard.xml
- Open it, and change the CodeBeside class into your custom class ([Namespace.CustomClass], library name).
- Next, you have to recreate the whole ImportUsersWizard, with a customized code in PerformImport function.
- Compile your library, and put into the Sitecore bin folder, you are good to go.
Hope this helps. Feel free to leave your comment if you don't understand ^_^