I'm really not an expert on BPMN or BPML, but I believe BPML was a proposed standard that never really for off the ground about 11 years ago. I guess there may be some vendors that have something they call BPML that they can import. So... the first thing you need is a detailed specification of what this "other tool" supports.
Now... you don't say whether you want to do this JSON > BPML client or server side. Client side you can handle the JSON with JS easily enough, or just use the GoJS data model directly.
Server side, you'll need a parser like Json.NET or (for Java) see How to parse JSON in Java. Once you have the JSON as objects, it's just a matter of traversing the graph and generating the appropriate BPML.