I think Coach Chad’s standard advice on the podcast on adding volume is to add 15 mins z2 at the end of a workout, and I don’t recall him differentiating between adding this to VO2Max workouts versus other types.
Although if you are specifically trying in this training block to work on VO2Max as a weakness, I’d be tempted to “save” the additional TSS, and try to squeeze in an additional 30 or 45 minute VO2Max workout later in the week, rather than adding z2.