Hi, I'm playing Pokemon Platinum in NO$GBA, and I was trying to get a Charmander. Of course, I just started, so I only have the Sinnoh Dex, not the National. If I use Pokesav to give myself a Charmander, what will happen? Will the game crash, since obviously, Charmander can't go into the Sinnoh Dex?
If you modify a Pokemon in your Box or Team, it would not register into your Pokedex. However, I don't know what'd happen if/when you evolve it.
I believe your Dex would simply update to the National Dex automatically if your edited Pokemon evolves. If you're that worried, activate the National Dex while in Pokesav.
The only way to hack a Charmander into the actual pokedex entry is to make an egg of Charmander and hatch it. Until you get the National Dex, his entry will not appear.
but will I still be able to use Charmander and its evolutions with no problem? Like, will the game crash?
If you have a Pokemon who's numbering isn't registered in the Sinnoh dex, that Pokemon's numbering will be displayed as "???". For example: Bulbasaur is # 001 in the National dex, when in the Sinnoh dex, it'll be # ???.
I've done this before. Don't worry, if you cheat in a non-Sinnoh Dex pokemon nothing should happen. Of course, you won't be able to check it and it's evolutions in the Dex until you get the National Dex. It should be okay to evolve it too.