The dining room tables that you buy for about $3000.00 set with hutch
seem to have a very thin clear coat on them.

True.

Does anyone know if it
is possible to add extra clear coat to make it more scratch
resistant.

Yep - I did exactly that. Didn't touch the legs or apron; just the
table top and visible edges.

Can you refinish the table by sanding and re-applying some
clear coat? I assume that most tables are wood veneer tops. Thanks

I wouldn't attempt to sand all the way through the existing finish,
for exactly the reason you stated - I'd be too worried about sanding
through the veneer. If you just want to add additional clearcoat, I'd
recommend sanding the existing finish VERY lightly with maybe 320 or
400 grit, and watch the corners - even with the fine grit, it'll be
easy to sand through (DAMHIKT). Anyway, that bit of roughness will
give the next coats something to grip. Then you can use polyurethane,
old-fashioned varnish, or some water-based variant, in a brushing,
wiping, or spraying variety. I used Behlen's "Rockhard" Tabletop
varnish. I'd do several coats of whatever you pick, sanding lightly
between each. If you have access to a woodworking store (i.e.
Rockler or Woodcraft), I'd go there and see what they have (or ask
them for a recommendation) rather than heading straight to a big-box
store to buy your finish.
I'd also recommend a finishing book (I have Flexner's) so you know a
little more of what you're dealing with, and this will give you some
ideas for how to finish to whatever sheen you want, maintenance, etc.
Good luck,
Andy