Hi,
Well, that's an interesting and very ambitious project.
However, you should expect that the time you will spend on it will be considerable, and of course you'll need to build the extension board, which in itself might cost you more than purchasing a ready-made OP4 STM32L daughterboard. ;/
Anyways, here is the way to proceed...
First, you need to read all these docs:
. GettingStartedARM and Ride docs
. All Open4 and CircleOS docs for the base and the two daughterboards.
. STM32 devices (L & F) docs (datasheet and reference manual)
. ST libraries docs
. temp and pressure sensors docs
Then, create a project for OP4_STM32G, copy the STM32L application code in it, take the code for low power and accesses to temp/pressure sensors from the STM32L CircleOS code, include it in your application and adapt it whenever possible... That's it!
Best Regards,
Vincent