1. Is it feasible to implement something like "User mode Linux" on
  NetBSD; id est, is it possible to create a NetBSD port to run on top
  of NetBSD? What would be the difficult things to do a port like that?

It should be just as possible as UML, so sure.  The tricky part is what
kind of virtualization and protection you get.  I'm not clear on how UML
works - it's not really virtualization, but lets you do some things you
can do with virtualization.

  2. If that is possible, would be feasible to implement the NetBSD on
  top of NetBSD port 100% portable? I mean, if NetBSD has been ported to
  run on top of NetBSD syscalls instead of direct hardware access, would
  be possible implement this port ONCE and compile the same source with
  no modification on every supported platform?

There are three broad classes of issues in portability.  One is
hardware (IO), but also processor memory management and things like the
sizes of integers.

This is surely a nontrivial amount of work.