If you use MAX or another app with inches etc. e.g. inches
Import the model or linework into Maya and scale it down to centimeters or by 1/25.4 and use Mayas default units (centimeters) reason for this is that other aspects of Maya intepret units as cm regardless of the units you specify. At the end of the modeling session in Maya just scale your scene up globally by 25.4 or 24.5? can't remember right off hand.
I've heard other users indicate that models exprted from Maya come in really small; reason is Maya exports as cm (75% sure on that theory only). It doesn't matter though.
I will write a mel script that allows users to input numeric fields in feet and inches at some point but for now you can use the (relative and explicit) coordinate entry in the status bar of Maya. For instance if you set it to relative or "Rel" and would like to move a sphere up 4'-6" you can type in for X Y Z
0 (12*4+6) 0
You could leave of the 0 for "Z" in this case and following basic algebra the 12*6 is evaluated first and then add 4 which will show up in the channel box as 50 units (centimeters actually)
Unfortunetly you can't input this into the attribute editors translate node unless you evaluate it as an expression and it doesn't work in the channel box either but it does work at the command line which would be
setAttr "nurbsSphere1.translateY" (12*4+6);
But you can do this in the channel box in 3 lengthy steps.
to get your feet translation type:
= 12 "enter" result = 12
= 4 "enter" result = 48
= 6 "enter" result = 54
So clearly the status lines' relative and explicit entry box or the command line is the best place for evaluating these type of inputs.
Hope this helps ya.