Neither client password nor resource owner credentials grant (ROCG) are directly supported by Spring Social (yet). But they are relatively easy to perform with a simple RestTemplate, since they only involve a REST call to the token endpoint with a half-dozen or so parameters.
The latest snapshot build have support for ROCG at the OAuth2Template level, but it is not yet supported at the connection framework level for purposes of creating a connection from the obtained access token. That is on my radar, but just is a few notches lower in priority than some other stuff, so I've not done it yet.
As for client credentials grant, it doesn't make as much sense to support it in the connection framework, as connections represent a 3-way relationship between a user, an API provider, and a client...but client credentials grant produces an access token that represents a 2-way relationship between an API provider and a client (no user involved). So, although it would make sense to support it at the OAuth2Template level, I don't see a need for it at the connection framework level. That is, unless you can make a compelling case for it.
Spring Social Project Lead