Yes, the X-E1 and X-Pro1 can both be configured this way by simply placing the camera into MF mode. The AE-L/AF-L button then becomes an AF button.
I assume the X100s works the same way, but I am not sure. You can check the X100s manual here: http://www.fujifilm.eu/uploads/tx_fujifilmproducts/downloads/X100S_manual_02.pdf
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