Picocalc has become pretty popular but it‘s still difficult to find basic hardware info….
Are (some of) the GPIO pins configurable as analog inputs? Or as analog outputs? Or will I need a A/D converter to masure a dc voltage?
yes, you need an external ADC. the Pico has one built in but it uses pins 26, 27 and 28 (29 if you want to measure VSYS). in the PicoCalc, 26 and 27 are the speaker outputs , and 28 isn’t wired anywhere.
there’s plenty of hardware info to be found in the repository: GitHub - clockworkpi/PicoCalc: A calculator like nothing you've ever seen before, right?
my mistake, pin 28 is actually one of the ones in the side GPIO headers, so you can use that for analog input. it’s just a matter of support in whatever firmware you’re using
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Yes, pin 28 is perfectly usable on the regular pico, things change a bit if you use a Pimoroni though, you`ll have to use pin 42 and switch pin 28 off, like;
SetPin gp28,off
SetPin GP42,ARAW
for instance 
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