Back in college I was told to not think too hard about Vos, as it's typically temperature dependent so you need to assume it's non-zero or will be in some circumstances and compensate with feedback anyway.
Technically you probably could do it externally in most cases but it would require a bunch of extra stuff, and be a pain, so usually it's best to use the stuff built into the amplifier itself.
https://www.electronicsurplus.com/national-semiconductor-cor...
But the potential advantage that building a chopper (or auto-zero) out of ordinary opamps would be that you don't need to source, order, and await specialized chips. A long-discontinued bolt-on auto-zero for a regular opamp has almost all the disadvantages of just buying an auto-zero opamp.