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.
I've seen designs that use a Chopper Op-Amp to actually auto-zero a power-OpAmp, effectively transferring the low Vos characteristics to a different OpAmp with completely different characteristics.
If the op-amp has a stable Vos I feel like maybe you could zero it by hand with a trimpot? You just need a button to short the inputs together while you're trimming it.
That's only rated good to 5 μV, according to the datasheet. You'd probably be better off with a non-auto-zero LM324B, whose worst-case Vos is rated at 3 μV IIRC. Of course it didn't exist at the time (01989) which is why National made the LMC669.
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.
