Hello DA Dev Team.
Please add SSL Support for Multi Domain & SNI to PureFTP
As far as I know this is already build in.
What is the problem you're having? I can use TLS (so with LE) on FTP with pure-ftpd.
If that is not working for you, it might be better to ask over at the forums.
You migh want to reply at the forums because I don't look here very often.