Well, I've been searching for a while around the MSDN documentation, questions here and other sites and even some books but I can't find any clear reason why there isn't any information about how to develop Add-Ins (VSTO) for SharePoint Designer 2010 and recent versions.
The little info I've found about it is from the book "Professional Microsoft Office SharePoint Designer 2007" which has a chapter about developing an Add-In to export Workflows and open them in Visual Studio, but the authors state that Microsoft doesn't support these officially.
Even knowing that, the authors create the Add-In using an unofficial template for the creation of the Visual Studio solution. So my question is:
It is still possible to develop a SharePoint Designer Add-In for versions upper to 2010?
If so, there is any available template that can be used to create the VS project?