How to get 110 from 240
It's easier to teach a violin player to play the bongo drums, than the other way 'round.  


You need to take those lights back and get universal-voltage lights that work on any/every voltage between 100 and 277.  (100 is Japan, also 110-10%, 277 is pole-neutral on 480 wye).  In this day and age there is no excuse for someone not selling universal voltage lights, it literally adds 2 cents to the cost of manufacture. 

Here's why converting the ceiling to 120V won't work. Several reasons. 

First, metal halide is already very efficient, and LEDs are less or not appreciably more efficient. LEDs are better aimed, but only if they are actually aimed (corn cob LEDs are not).  That means you need to deliver potentially the same amount of total power.  When you cut voltage in half, you must double current.  It's easy to insulate wires for higher voltage, they're all already insulated for 600V, but to increase current you must install wires twice as thick! 

Realistically that means breaking it into several circuits, as you still have 20/30A caps on lighting circuits. This is becoming quite an undertaking.  Now one option is to add a neutral wire and rewire the ceiling as "multi-wire branch circuits".  However this is complex and easy to botch.  

Wire colors must be correct.  If they are cabled in Romex, chances are the white wire has been used for a hot, and it can be easily re-tasked to be a neutral.  That is easy enough.   However if this is in conduit, the two 240V wires are both hots, so they will both be colored wires.   In conduit, you must use natively white wires for neutral! That means pulling additional wires. 

Lastly and mostly, you mentioned that your power is 3-phase.  There are several arrangements of 240V 3-phase.  You are thinking any random phase wire will be 120V from neutral.  That is not true, in fact, it's impossible. In regular wye, it'll be 139V from neutral.  In wild-leg, it'll be either 120 or 208 from neutral, and they are supposed to use orange for the 208V wire, but you can't count on that, especially in circuits where they never expected to even use neutral.

Anyway, the various dialects of 3-phase are wizards-only territory.   You've been fairly candid about your skill level.  I agree that changing lamps is well within your range.  However the engineering needed to deal with the above is considerable, and it would be quite complex for us to walk you through it all, and even then, we could miss something important.

Bongo drum rule: Take the lights back.