Using Custom Database for WordPress login

** Single login for WordPress, Mobile App**

Hey Guys
I have a WordPress blog & have an Android & iOS App for the blog. Now I need to add My account feature into the app & wondering what’s the best way to get it done which doesn’t affect my future plans.

I Plan to use a custom database for user management for my iOS app & Android app (For the same blog). Here is the question & any kind of your advice would be appreciated:

Can I use custom database for user management of my WordPress blog.
If not, can I sync user table to & fro from custom database to existing WordPress database & vice-versa.

I need Single sign-on for all the platform (I also have a discourse forum) & if you can point me to right direction, it would be of great help.