How do you place extended units for extended players in a map?

1. Place the unit for any player.
2. Select the unit, right-click and hit "Properties"
3. There should be an owner field where you can specify a number. Specify anything between 0-255

Assuming you mean players 9-12: You can't create them via triggers, but you can use the "Give Unit" action to give to them.
Assuming you mean players 13+: You can't create them or give to/from them via triggers. You must pre-place them.

Please note that pre-placing units owned by players 13 and up is known to be unstable and have unusual (albeit predictable) effects on your game.

These effects are subject to change during patch updates, but Blizzard doesn't seem to be planning to release any new patches, so this is more of a fun fact than anything.

You'd need a better editor, like starforge or SCMdraft to do so