According to Germany, It IS possible to use NPN high speed inputs.

Using the ifm templates for the 32 bit controller:
1) Configure the input as a low input (turn off the high, turn on the low config bit)
2) Do NOT set the high speed configuration bit.
3) Use the appropriate high speed function block (ex: HIGH_SPEED_COUNTER) and assign to the channel corresponding to the input.

This should allow you to use the low input in high speed mode.

NOTE: On the 32 bit controller, LOW inputs are reversed. When the input is off, you will recieve a logical TRUE in the processor. When the input is ON you will recieve a logical FALSE in the processor. This may have an effect on the PERIOD function. Needs to be tested.