The procedure for registering new suffixes is now defined in http://trac.tools.ietf.org/html/draft-ietf-appsawg-media-type-regs-14#section-6.
“+json” will be defined in a separate document; right now: http://trac.tools.ietf.org/html/draft-ietf-appsawg-media-type-suffix-regs-02#section-3.1
And no, you are not supposed to have multiple subtypes there.