Put your new rr-pair into its own directory. Right-click the service image folder in your Workstation, and select "Create new service image; from request-response pairs". The generation wizard starts.
Choose the name of your already-existing service image. If you choose the name correctly, a radio button will become available directly under your image name, called "merge into". Select this, (double-check, because the other option is "replace", and that will overwrite the service image) and the rr-pair will be inserted into your service image, rather than creating a new one.
I suggest you backup the previous VSI file before you start, so you can try it a couple of times and verify that it worked correctly.