Hi Luiz!
This post does the string conversion
Note that the end goal of this post was to get it into millisecond form. Here you just want the string, so use this expression:
set usDate to [[ d.substring(d.indexOf('/') + 1, d.lastIndexOf('/')).concat('/').concat(d.substring(0,d.indexOf('/'))).concat('/').concat(d.substring(d.lastIndexOf('/') + 1, d.length)) ]]
…assuming d is the local variable referring to your date in DD/MM/YYYY format.