@stuart-auld welcome to the FusionAuth community!
There is no within FusionAuth to modify a webhook body.
This may be a feature of interest to the community; I'd encourage you to file a feature request with more details, but we could possibly run a lambda run every time a webhook runs to transform the webhook output. (There's probably scope for adding lambdas in lots of places; here's another one.)
Until that happens, however, you can send the webhook to a process running on a server or using something like google cloud functions or AWS lambda to transform the webhook output.
