If I understand you correct, you don't want to convert your database, but want NodeBB to work with users, that still will be in your existing database (MS-SQL).
It's possible, by creating your own database driver for NodeBB.
But, actually, it's not so easy. If you don't have high skill in programming, it's near to impossible.
Is there a way to set the default daily digest settings for all current users and any future registered users for the site? The default seems to be off but, I would like to change that to weekly if it is possible.