Altering the design of helmet

I need to alter the design of this helmet file. I need to make the area around the chin strap slots thicker. The two red lines is where I need to make it thicker. I imported a step file and it is a sealed polysurface. Can I create a edit cage just around the chin strap slots?

Yes you can have the cage only effect part of a model…it may take some experimentation to get it to work nicely but it can be done…