Number to text

An old version of the Acrobat SDK had an example for that, but unless you have that version (I think it was back in the days of Acrobat 4), you won't be able to download that anymore. However, you can use any JavaScript implementation of this feature that only uses core language features. Take for example a look at this Stack Overflow discussion: JavaScript numbers to Words - Stack Overflow

I would recommend that you save the "number to words" functionality in a document level script and then call it from your field's custom calculation script.

Do you mean 12 to "twelve" for example?