-I want to add a relief zipper.

Can this be done by someone handy enough to replace the latex gaskets,
or do you recommend having it done by a company who specializes in that?

It's possible to do it yourself if you have a commercial sewing machine
and you can find the zipper. As much as I like DIY solutions, this is
one job that may be better off being done by a professional.

-I want to add neoprene gaskets over the latex ones, in part to prevent
scratch and UV damage as well as to keep the water out even better.
Anyone know of a company in Europe who does that kind of alteration?
I can do it myself, cutting up an old thin wetsuit and sewing on the
gaskets myself. Still, I prefer it done by someone who knows what
they're doing... :-)

You could glue cuffs one with Aquaseal or a similar adhesive/sealant.

-I was thinking about having latex socks added to it instead of gaskets.
I remember reading on RBP (probably years ago) that it is better for the
circulation as well as making certain that you keep your feet dry. I was
thinking about wearing booties over them.

Good idea. Latex socks are vastly superior to ankle seals.

Anyone know where I can find those latex socks in Europe (or online?) ?

www.ossystems.com

Latex seals are easy to install yourself. Kokatat has instructions on
their site and I've got pic and instructions in a Webshots album at:

http://community.webshots.com/user/brian_nystrom-reg

It on the second page under "Drysuit Repair".