Is it possible to use Dual-Slot Mode before beating the champion for Pokemon that are available in the dex normally?

Question

I've looked about, and all I find is info stating that the Champion must be beaten before you can use Dual-Slot Mode for Gen III Pokemon. I haven't found a source stating if it's possible to get Pokemon in the Sinnoh Pokedex by this way. I'm trying to get Gengar in the Old Chateau if it's a one time case where it does/doesn't work there.

Comments

You can get the national Pokedex before beating the champion.

Answer 1

Yes. You can use it as soon as Pal Park is opened, which can be done before becoming Champion.