Handling requests from non-mesh services

Hi, I want to migrate small part of existing services to Istio service mesh(they are on separate namespace). Some of non-mesh services communicate with these services.

One of my goals is to be able to conduct ab testing of services in mesh. For example, some non-mesh service(let’s call it A) makes request to mesh service(B). I would like, based on request headers, map this request to correct instance of service B.

Is it possible to implement this? Or I have to add service A to mesh?