First of all: we transform it into extended CNF, not pure CNF: we keep the unary rules.
Now, to answer your question: in this very case, yes; but this is a very special case since both X and Z are NEW non-terminals. I prefer you don't bother and remember only the simplest rule, which always work: introduce a NEW non-terminal FOR EACH of your transformations.
The problem would be when using non-terminals which already represent something else. For instance with
S -> NP VP
S -> NP VP PNP
it is
VERY WRONG to change second rule into
S -> S PNP
because then you in fact introduced infinitely many new parses like:
NP VP PNP PNP
NP VP PNP PNP PNP
Makes sense?