I want to add custom variables to "Insert Variable" popup of Magento's WYSIWYG editor (TinyMCE)
Searched the internet but couldn't find any solution, is that possible? Any one?
You can add a custom variable in your Magento backend:
System -> Custom Variable -> Add New Variable
Here you can add a HTML or plain text variable.
Variable code is identifier. Variable name what will be displayed.
For adding custom variables programmatically, see this blog:
http://inchoo.net/ecommerce/magento/injecting-variables-into-a-magento-cms-static-block/