How to provide ventilation on NEMA 12 Arcshield enclosure with help of fans?
There are no NEMA 12 fan options for PowerFlex VFDs in the "100ms" version of ArcShield MCCs, but there are in the "Device Limited" version. Device Limited ArcShield MCCs mean the MCC can have a maximum of 1200A bus and the main must be part of the MCC. 

If you have an MCC at 1600A or larger, then you are limited as to which VFD options are available to you and there are no options for NEMA 12 vented VFDs, so that limits you to I think 40A (25HP @ 480V) or 34A (20HP) if there is a reactor, which is the largest non-vented VFD in a NEMA 12 package. 

What I do if the project needs larger VFDs in NEMA 12 and needs to be Arc Resistant is to split up the MCCs into 1200A or smaller segments so that I can use Device Limited, which allows for vented VFDs in NEMA 12.

But to answer your specific question, you cannot add anything to the front doors of an ArcShield MCC in the field without violating the Arc Resistant certification of it.

Side note: You mentioned "NEMA" so I am assuming you are discussing the 2100 Series NEMA MCCs, not the IEC version 2500 series. Those options may be different for IEC.