Convert the input of an ATX power supply from 110 V AC to 11-14 V DC
Yes, it is theoretically possible.  But by the time the conversion is done, most of the ATX power supply would have been redesigned.  The hard part of it.  The hardest change would be the transformer.  To give an idea about complexity of an ATX power supply, here's a reference design.

There exist ATX power supplies which run from low voltage DC.  These are used in vehicles and other field applications (example).  Look for them.

DC to AC inverter could be another option, of course.