We use the System.Windows.Forms.Shortcut enum which does not include that in the enum.  So to get something like this to happen you can handle the forms KeyDown and check the keys and then do a call to a function that does the same code that the button click would have done. 

For instance:

        private void Form1_KeyDown(object sender, KeyEventArgs e)
        {
            if (e.KeyCode == Keys.Tab && e.Control && e.Shift)
            {
                //perform the action you like
                MyButtonToolClickCode();
            }
        }

        private void MyButtonToolClickCode()
        {
            // MyButton ToolClick code 
        }

        private void ultraToolbarsManager1_ToolClick(object sender, ToolClickEventArgs e)
        {
            if (e.Tool.Key == "ButtonTool1")
            {
                MyButtonToolClickCode();
            }
        }