I've not tried this personally but I do believe you can do this. In addition to compiling mongodb with ssl, or purchasing one of the MongoDb subscriptions that support SSL you will need to run with the following option:
--sslMode
set to preferSSL. This will use SSL for inter-server communications but allow both SSL and non-SSL for other connections:
http://docs.mongodb.org/manual/reference/program/mongod/
That of course is all in addition to the other configuration settings required for running with SSL:
http://docs.mongodb.org/manual/tutorial/configure-ssl/
Note that this is new for version 2.6. I don't have a version of mongodb compiled with ssl support so it's not been tested by me.