Can FBA be added to existing Site Collection?

Hello SharePoint Fam,

I have been asked to possibly configure a way for non AD/outside users to access our SharePoint environment.  I am leaning towards Forms based authentication and was wondering if this is something that has to be done before  a site collectoin is created or can I use my existing site collection to allow FBA??  Totally want to keep AD authentication for our network/domain users but also need access for outside users. Really hope I can just add this authentication to existing environment/sites.

Thanks n advance