You can't. The environment is a list of strings that is passed
as an argument to the execve() system call, so that some data of
the original command can survive into the new execed command.

If you want your shell memory to be updated, it must be
something initiated by your shell. Typically a process can't
alter the memory of another process, that would cause all sorts
of reliability and security issues.

What you can do is something like:

eval `executable`

and manage to have "executable" display some csh code. But the
fact that csh has got a number of issues wrt quoting is going to
make it difficult to do reliably.

Do you have to use csh?

--
Stéphane