dump contents & missing functions

Is it possible to make a schema change to a dumped backup?  I've
un-tarred the file and edited toc.dat and restore.sql, but when I
attempt to pg_restore the re-tarred file, I get an archiver out of
memory error.
This whole issue arose when I set a default in a table row equal to a
value that a plpgsql function returns (ie the table default =
functionname(variable)).  However, after exploration of the dump files,
it appears that pg_restore tries to restore the tables before it
restores functions...hence I get an error upon restore of the original
dump, because the function does not yet exist.  Does anyone know if the
ordering of the restore is in fact the problem?  Any input is appreciated!

Kris