Please how can i add a nondesign component with my model 3d

Please how can i add a nondesign component with my model 3d ( i want that the thickness equal to 4mm of the nondesign part )? 

i want get please a model like picture number 1 ?

you find attached the file.FEM of the part if you want add it by you 

 

Thank you a lot 

 

Best Regard 

 

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