Actually, I'm pretty sure you can't make the Mongols and Timurids spawn like the BI mod. What you can do, and what I think Callatian is suggesting, is change the campaign map so that it starts at a later date and have the M & T playable from the beginning.

I am happy to be proved wrong that the same technique used in my All Factions mod for BI can be used for M2TW because it was the first thing I tried.