First, concerning the subject of this thread: 2ZZ and 2AZ are very, very, very different engines so the chance of the 6-speed from the 2ZZ fitting on our cars is less than slim.
And while we are at it, let's do some math. Let's assume that adding a 6th gear will add 2mpg to our existing EPA value of 29mpg bringin our car's EPA highway gas mileage closer to 31mpg, and let's assume that you drive 15,000 miles a year, mostly highway. So with 29mpg you would have used 517 gallons of regular gas, at 31mpg the figure would have dropped to 483 gallons, 34 galons saved in a year, and even at $2.50/gallon the savings are $85/year.
Simplest tranny swap when everything fits will probably cost around $300 in labor, plus another $1000 for the tranny from the junk yard, that's if everything fits. So your tranny swap will pay itself off in measly 15 years.
All this is true if:
1) You only drive highway
2) You drive 15,000 miles a year
3) Tranny swap will result in at least 2mpg efficiency increase
4) Gas prices rise to $2.50/gal
5) Tranny swap will cost you $1300 including the tranny.
Since most of these points are a (sometimes small, sometimes huge) stretch we can come to a surprising conclusion - leave the damn thing alone.