modifying an .stl file

[chunk07]

I have an .stl file I need to modify. I don't have the Fusion file for the model.I need to modify 2 areas to add width. I cannot use scale as there are critical dimensions that would also be effected.

I have successfully converted the model to solid, created offset planes, and split the model at the planes. I can hide the 2 pieces and use extrude to widen the center part as needed.

My question is how do I re-assemble the 3 parts to include the new width?

[HughesTooling]

Combine?