You cannot inject/slipstream custom drivers during ESXi installation. That was possible during ESX 4.x installations, but has never been true with ESXi 4.x/5.x installations to my knowledge.
The best you can do in this case is use Image Builder (as pointed out in the KB some people have mentioned here already) to create a custom ESXi 5.1 image that includes your missing drivers, if they are even available.
I would check the Hardware Compatibility Matrix to make sure your driver is supported under ESXi 5.1.