It is not possible to use a different membership provider with mojoPortal, but it is potentially possible to use the mojoPortal membership provider in other applications.
Internally mojoPortal uses its own business objects for user and roles so it cannot use an external memberhsip provider becuase the MembershipProvider API does not provide all that we need and an external one would not map to our business objects.
We implement the API for MembershipProvider but that is really only useful for things like the ASP.NET loginControl, AuthenticationService, CreateUserWizard, and PasswordRecovery controls which bind to the membership provider.
None of this is likely to ever change, so the answer to this question isn't likely to ever change.
Hope that helps,
Joe