HestiaPi Classic and I2C: access to pins without removing the shield?

Checkout the pinout here

and make sure you use different I2C address than 0x76 for included sensor. Multiple I2C devices can use the same channel.
You would need to solder wires for your sensor. By the way, the included BME280 sensor does measure humidity too but I guess you want a second humidity sensor for another reason. Feel free to elaborate.