Hi, I'm trying to do this simple thing but I can't do it yet. I have read some documentation and tried several ways and nothing, so I'd appreciate specific details to perform this task.
Maybe you can provide little more details on what exactly "the task" is? Adding a column with asp:TextBox in password mode? If so, just use a GridTemplateColumn and place the needed controls in the appropriate templates.
http://www.telerik.com/help/aspnet-ajax/grdcolumntypes.html and scroll down to the TemplateColumn section.