You can perhaps leverage a third party rest client library like 'Jersey' and call your external Rest service but I am not sure if the amount of customization is worth the experimentation needed for this to work in 6.1sp2. On the other hand, Connect-HTTP can been used to call external rest service in 5.5Re: How to call REST Service from PRPC5.5
I strongly recommend upgrading to Pega 7 where this is supported OTB.
Is it possible to parse json string from http response without any third party library in 6.1SP2? If it does, could you please share the implementation steps? If it has to be done with third party lib, could you please share the steps of how to do that?