Can I use a MCP23017 to connect multiple RC522 modules?
Sure, you can use IO expanders for the chip-select pins of SPI devices. You will have to modify the device libraries to support them, though, because no one is going to do that by default unless it's a very very specific library.
However you will probably not have much luck with your project anyway, since you will more than likely run into problems with fan-out and bus capacitance before you even remotely approach 64 devices on your bus.
You would need to be adding extra bus drivers and partition your bus to reduce and spread the load - at which point it would be simpler and cheaper to just use multiple microcontrollers with just a few RC522s one each.