In django-allauth, is it possible to use an existing provider app and customize it so that I can connect a currently unsupported site like IFTTT.com?
Yes. You can see all the supported providers at https://github.com/pennersr/django-allauth/tree/master/allauth/socialaccount/providers you would simply copy a directory and adapt it for your provider (ifttt). Feel free to submit it as a PR afterwards.