How can I measure CR2032 RTC battery voltage in a microprocessor system?
I also want to use some mosfet to connect resistor as load (around 10-20kOhm) at the moment of measurement to make measurements more reliable. Maybe I could just connect 10k-20k resistor directly to ADC and connect that (on high side) by mosfet when I take measurement?

I question the need for the 10-20 kOhm resistor. But the high side switch between ADC and battery should be the only thing you need. I don't see why you need a voltage buffer for a battery. When the FET is off, leakage through the FET should be minimal, so minimal draw on the CR2032. With no buffer, there is also no draw on VCC. What is the VREF for the ADC? Is it in all cases, higher than the voltage of the CR2032? The ADC cannot measure any voltage higher than VREF.



simulate this circuit &ndash; Schematic created using CircuitLab
For M2, a BSS84 may be a good choice (but double check everything).
For M1, a BSS138 is probably the best choice (but double-check).
For both transistors, there are many parts available that will work.
It would probably be a good idea to put a small capacitor on the ADC input to help with noise filtering. It might not be needed, in which case you can leave the component off. But if you need it, and the footprint is not on your PCB, then you have no options.
The cap, if used, will steal a tiny bit of charge from the battery every time you turn on M2, but that should be negligible.
I chose R2 and R1 somewhat randomly. You can probably use different values if you want. Anywhere from like 1k to around 1M will probably work.
R3 is probably not needed, but people frequently recommend a series resistor for the gate drive of any FET, even these small low-power FET's. So I put it in there just to avoid their comments.