incorporate a multimeter into the Pinecil

[lalisa12]

I had the same idea.
Below I will share my thoughts after a quick datasheet read for V1.

You might be able to repurpose the tip sensing mechanism although the measurement range would go from 0-0.01V without further voltage deviders. After that the Opamp will Saturate.
And I don't know if the driver for the tip would interfere fnaf 2.

Otherwise you defenitly need to use an addon board there are at least 3 pins routed to the usb port which can be used with ADCs.

Without additional circuitry you would only be able to measure 0-3.3v (presumably, didn't find any information on VREF and the QFN36 package).

Using the usb Port you could even create an osciloscope and display it on a Phone or PC.
12Mbit/s /16Bit/sample = 750ksamples or 1Msample/s with 12bit for a single channel minus usb overhead.