I would like to know if I can convert monsters from 3.x editions (I already figured out how to convert monsters from the 1ed) into 2ed monsters.
For 3.X monsters:
Keep the HP and HD as listed in 3E. Assign the THAC-0 based upon the HD. Convert spells by name; most early 3E stuff is direct from 2E.
Convert saves by feel; level should be same as HD. Combat monsters should be as fighter; spell-users as MU or Cleric. Sneaky poisoners as Thief, etc.
Assign asterisks as per the DMG guidelines.
Note: There was a conversion manual for characters from 2E to 3E. It's quite possible to work backwards with it for converting NPCs, but it's much easier to just grab the atts, and lose 1 point of stats per 4 levels, and use same class and level, assigning proficiencies by feel.