If you are asking if there is default functionality built into PBCS to convert CSV to JSON then the answer is no.
You could always write a custom script to convert your CSV file to JSON, it all depends on your scripting language, not sure why you require it in JSON format.
The reason for this is that I need to do a delta comparison between the new metamodel that I'm about to load into PBCS to what I have loaded at the moment. I could use the REST API to get the members in JSON format, but then I still have to convert my CSV to JSON as well. Do you have any suggestion?
I am not sure why you need to convert the CSV file to JSON but there are different options available, you could use the REST API to export/download the current metadata and then compare.
You could use the Get Member REST API to check if a member exist but if you have a lot of members then that is a lot of REST calls.
There is also the DM REST API resource (not documented) to return a full dimension and members, which you could iterate through compare to your source.
Though if you keep a master of the current members and manage that when new members are going to loaded then it would be more of a process to control than having to keep checking PBCS.
Is there any "un-official" place where I can find the DM REST API ?