Of course, you can put an Usercontrol anywhere in the Grid.
Say you want to place the Grid at Row 1, Column 1, on SelectionChanged of Tree node, use :
yourGrd.Children.Add(yourusercontrol);
Grid.SetRow(yourusercontrol, 1);
Grid.SetColumn(yourusercontrol, 1);
This will work. Your usercontrol will be placed on correct location.