The Dropbox Java SDK doesn't officially support changing the port or protocol (all calls have to be done over TLS so the port is always 443), but I'll pass this along as a feature request. I can't promise if or when that might be implemented though.
The closest thing the Java SDK offers is the ability to set custom hosts via a DbxHost, using this DbxClientV2 constructor. I don't have any sample code for that. I don't believe it would let you change the protocol or port though.