Although quite oddly phrased, the OP wants:
Single Sign On (SSO).
Q2A to work with existing user database.
What the OP did not specify is the nature of existing platform/CMS etc.
If it is a custom user feed, nobody can help you without knowing how your setup is.
If it's MODX, see my tutorials/demo.
If it's WordPress, there is an official bridge for that.