Do you want to create a partline that is ruled around the edge perimeter or do you want to extend the interior surface? If the latter, that's going to be tough since the part was not model correctly and there is a degeneracy in two quadrants of the face (see attached) - faces like this are impossible to extend.
Best of luck.
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I would look at the Ruled surface feature in SW. You could also do something with projecting edges onto offset planes and using extruded surfaces, if you want to simplify the shut off surfaces as much as possible.
But unless I'm not seeing something right, I don't think the red surface is necessarily at the PL, is it? It looks like the fillet around the edge has to be split in some places. I'm basing that on using the Front plane as the direction of pull. It's possible that the actual direction of pull is determined in some other way, such as perpendicular to the axis of the through holes. Yes, on looking at it that way, it makes more sense.
Anyway, the Ruled surface is usually the way to go.
Thanks everyone for their help. I really appreciate it.