Photoshop has had a filter to remove dust and scratches for a long time. Is there a simple way to do the reverse and ADD the appearance of dust and scratches?

Get a bunch of dark specks on white. Put it in an upper layer set to multiply.

Get a bunch of light specks on black. Put it in an upper layer set to screen.

Get a bunch of both dark and light specks on 50% gray. Put it on an upper layer set to overlay.

Also get to know the blend-if sliders. They can be handy.

Do a Google image search on "scratches texture" if you don't want to make and scan your own.

https://www.google.com/webhp?sourceid=c … %20texture

Easiest is to use either On1 Photo RAW or Alien Skin Exposure X2 that have those filters built into them.  You wouldn't need PS either as they are stand-alone programs and also operate as plug-ins to PS or LR both.  They provide a lot more variety than running within PS too when used as stand-alones and using their own layer and masking system.

Alien Skin Exposure has a good look but it also has a variety problem. There are only so many scratch patterns and the number isn't that high.

Or get the now FREE Nik plugin package, with the Analog Effects Pro 2and Silver Effects plugins.

https://www.google.com/nikcollection/