Can I scan barcodes using my Newton?

Yes.

Specifically, you can the InPath Mobile Wand 400. This is a rebadged or customized version of the HP SmartWand 8000 series — probably the HPSW-8200. The driver was written by Victor Rehorst (with assistance from Daniel Padilla.)

http://old.chuma.org/newton/inpath/

Announcement post on NTLK:

https://marc.info/?l=newtontalk&m=105538458228826&w=2