Need to work in negative Z axis

Hello Guys, I Just started with LInuxCNC, my CAD/CAM soft is fusion 360. So far I was workin on mach3 and alway pointed my Z axis Up, baze my material Z=0 on a table and was workin in material in positive Z.Now i Have my Upper Z limit smitch set as "0" So it creates an urge to work in negative Z sonstantly, and need to set material Z=0 on top of workpiece. Unnfortunately Fusion does not have independent axis turning.https://ibb.co/JpmP4WC

I don't understand the problem?

Is it normal to work in Z negative space. Is it standard to do this.
Z moving up towards the sky should move positive.

Whether you set Z0 at the top of the workpiece or the bottom.of the workpiece is up to you. Sometimes one is better than the other.
(This is work coordinates / G54)

By default I use Z0 at top of workpiece.

For example:
20mm thick workpiece.
Z0 is top of material
Bottom of material is Z-20

Fusion will allow you to have whatever axis orientation you want.

I don't see a problem. I just set the axis origin in fusion in the setup dialog, and everything just works. Z should still be pointing up, if not then I usually select x and y axis in the dialog and make sure x is to right and y is to the back (there are button to reverse both). If you don't have lines that match, then I just create a sketch with those directions in Design and then use those line in setup.