If you want to preserve the basic capability of the system (torque and cadence and multiple assist levels), but add a throttle, and it can't be done in the OEM system, then the "simplest" effective way I can think of is to replace the OEM controller/display with a Cycle Analyst v3 (to interpret the sensors and throttle and provide a throttle to run the controller), and a suitable controller to run the existing motor.
It's likely the baserunner or phaserunner could do it well enough (people have used these to run the BBSxx middrive systems), if you need something small and easily integrated into the appearance of the bike.
Assist levels can be done either with the CA's presets or with the Digiaux control, or with the PR or BR's level controls (via a separate typical ebike display type). If you don't like the look of the CA, it can either be placed somewhere out of sight, or even be modified to not use a display, and just use the tiny main CA board inside some other enclosed part of the bike, doing all settings changes via the serial-USB and computer-based setup program.
I don't remember if the battery has a custom BMS that won't operate without the OEM system (communications between them). If so, you could replace the BMS in it with a "generic" type, like the JBD smartbms that lets you monitor it via bluetooth on a phone app. (I think it is only that the bike won't operate without the OEM battery, the battery probably works with just it's main plus and minus connected).
That's just a general idea; you'd have a bunch of details to work out.
For instance:
If for some reason the existing motor can't be used without it's original controller, you could use a hubmotor; the existing system would have to remain in place because your frame requires it if you want pedals.
If the built in torque sensor or cadence sensor can't be used without the OEM controller, you might have to use an external crankshaft-mounted cadence sensor; I don't know any good options for a torque sensor you could add easily. There are dropout-based torque sensors, but they require modifying your frame. (the CA can use a number of types of torque sensors; as long as it has a compatible voltage range the CA can be set to read).
Buuut: You could use all these things on a completely separate bike and preserve your existing bike to work the way you use it now, as a backup / spare, or to resell.
Whatever you do to modify the system is likely to involve very major changes to it, possibly even requiring cutting holes/etc in the drive unit, depending on exactly what you end up changing.
