Can I put a Tuscany handle and backpate on an exsisting Moen mixing valve?

Are you asking about a Moen Tuscan Bronze trim kit for a shower valve? If so, it may depend on what valve you have. Does the handle pull out then turn left and right, or does the handle only turn countercloswise for cold/warm/hot? If the latter, you should be able to use the Moen Tuscan Bronze trim kit (faceplate, handle, escutcheon, shower head). They may even sell them for the pull/turn style valve.

If in doubt, call MOEN tall free number and they tell you.

Call Moen U.S. 1-800-Buy-Moen (1-800-289-6636)
8:00 am to 8:00 pm EST Monday through Friday
9:00 am to 5:30 pm EST Saturday
Closed on Sundays and all nationally observed holidays.

Hope that helps / Milo