Dear Loic,
The forum topic you linked to provides a lot of guidance for introducing a distributed-springs foundation model into FAST v7 through the user-routine UserTwrLd(). As discussed in that topic, the provided UserTwrLd() could be modified for p-y curves in place of simple linear springs. Using your own UserTwrLd() routine will require a recompile of FAST v7 with your UserTwrLd() routine.
More guidance for building a FAST v7 model with a distributed spring or p-y foundation can be found in the following forum topic: Natural frequency of NREL DS monotower in OC3..
Best regards,