Hello,
Unfortunately, this kind of progress bar cannot be created by using Outlook Object Model.
I tried to use CommandBar.Controls.Add method to add an ActiveX control by setting the first argument Office.MsoControlType.msoControlActiveX. However it did not work. We can create a new progress bar on the task pane, it is a little complicated [See: this link]. For task pane, please refer to this thread: Refresh the Custom task Pane in Outlook.
Here are some resources which may interest you: http://www.outlookcode.com/codedetail.aspx?id=1077 and http://www.outlookcode.com/threads.aspx?forumid=1&messageid=2149, which is related VBA rather VSTO.
Best regards,
Bessie
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