Hi Andrew,
Trimble Catalyst allows you to use any NTRIP caster.
To do so you need to install Trimble Mobile Manager; after TMM installed the Catalyst service you can configure custom correction settings.
This will task Catalyst to use the NTRIP mount point as defined in the custom settings; I'm assuming you then want to share the position via mock locations so it can be consumed by other apps.
Note that you still need a valid Precision subscription to make full use of the custom NTRIP caster.
Regards
Nico