Is it possible to share on the LAN a scanner connected to a PC on USB
port?
Define "share on the LAN" more precisely. All scanner applications
(like Vuescan or xsane or Omnipage) expect that the scanner is directly
connected via USB/SCSI/parport to the computer that the scanner
application is running on. The scanner application has to talk to the
scanner directly for various reasons.
It's possible to use the Windows TightVNCServer or the Linux KDE Desktop
Sharing tool to export the display of the machine the scanner's
connected to--that might provide a decent alternative for you. Whatever
scanning program you're using should, of course, save its output files
to a directory that's shared via Samba or Windows File Sharing. It's
not perfect, but it just might work.