The 5th Amendment of the US Constitution specifically permits eminent domain with "just compensation" so yes an American city could take a large chunk of its private real estate under eminent domain and put it to "public use". Converting vacant office buildings into homeless shelters would absolutely count as a "public use".

Buying some of the most expensive real estate in the world with “just compensation” seems like it’s more than a stroke of a pen.

Further, the court cases this will bring will take _forever_ because one side will be extremely well funded and the other side will be the city of San Francisco. Not the stroke of a pen.

Boston has done it, I worked with an affordable housing group that worked on this stuff. The difference was these were largely undeveloped lots, not office buildings.

That seems like a big distinction…

I'm not sure if that was sarcasm (especially the part with needle exchange), but it is San Francisco so yes, it could.

I would rather free housing go to people in need who work for a living

Why not? Theyve laid waste to hundreds of thousands of homes to build freeways with a stroke of a pen before.