Customizing the Excel Toolbox control

I am writing a VB add-in for Excel and want to customize Excel's control toolbox, (not the VB toolbox) with buttons representing some of my own controls. I want the user to get the same behaviour as if they picked one of the predefined buttons on the toolbox, i.e. pick the icon , select a point in Excel and the control is embedded. The only way I can see for the user to insert a control, (rather than an object), is to click the "More Controls" button and scroll through the list of controls.

Can anyone help ?