EXTEND SURFACE FOR CNC MILLING

Hi,

I'm working on a project where I will be milling EPS formwork of a shell structure. Long story short, the formwork will be used as a temporary support for 3D printing concrete ontop of.

What I'm stuck in is trying to extend the boundary of the surface sO that I will mill a surface larger than the print area (the attached images explain it better). Need to do this mostly so that I can easily orient the milled mould properly in physical space when it comes to printing.

Is there anyway to untrim the surface so that it extends out to a boundary without losing it's orginal shape? (It's important that the original shell structure geometry is maintained since printer paths are based off it)