Hmm, I don't think it can be done easily :-( The support
provided by Eclipse (and therefore Flex Builder) to modify the
toolbars is under Window > Perspective > Customize
Perspective (then click the Commands tab) -- but it is not
fine-grained control on the level of adding any individual command;
instead, there are "command groups." You can add or remove entire
command groups. I just looked, and I don't see one with Undo on it.
It can be done by writing an Eclipse/FlexBuilder plugin, but
that's a non-trivial task...