to my knowledge ADDT is a Kollection 3.7.1. compatibility release with no new features or major improvements on previous code -- but I wonder what "bug" you´re referring to ?
in alternative, someone have used linecraft extension "PHP Captcha Object" with nextensio form ?
as far as I can see, an external extension that´s supposed to replace a certain component like ADDT´s native captcha would require being able to integrate itself in ADDT´s general form validation routines, means
(like any other ADDT form validation component) to access "shared" functions located somewhere in ADDT´s "includes" folder -- maybe someone else knows better, but I strongly doubt you can "hook in" an external form validation extension without having to tweak ADDT´s form validation libraries. The same restriction would normally apply to other "frameworks" in return -- all of them are sort of "closed systems" providing a defined set of functionality using shared libraries