With #4085 and ec6712f the "extend" module was introduced in the package but the UCI doesn't know anything about it.
So i would suggest to extend the UCI with the possibility to configure the extend module.
As the snmpd grumbles about the exec line in the config:
Error: WARNING: This output format is not valid, and is only retained for backward compatibility - Please consider using the 'extend' directive instead
this should maybe replace the exec in general.
So atm i didn't make an PR but i want to share the following suggestion (don't know if this is the right way):