So if I understand, I have the model I wish to print. I place the new object where I want to change things.

So I end up with three STLs:
1: A file with the object AND the block where changes will be applied. This will never be sliced or printed.
2: A file with JUST the object I am printing.
3: A file with JUST the object I am using as the modifier mesh.

So I open file 2 as per normal. Then got to settings and load the modifer file 3 and make settings changes.

Then slice and print as per normal.

Have I got that right? :-)

Learn something every day.