There is limited customization you can do which just amounts to changing which is your 'pointer' finger. That requires editing the finger bone configuration of your avatar. Full hand animation support for the Valve Index is not available 'yet'. There are some animation files in the Creator Tools related to the hands but I have not played with them.
If it's small adjustments you need to make; like how far the fingers curl or the resting position of the thumb I'd recommend tweaking your bone and muscle settings; during the humanoid avatar set-up (FBX > Rig > Set to Humanoid > Configure)
Unity has a nice breakdown video of some of the settings you can change; including the bend ranges on joints here: