Geez! How cheap can you be? Get a slot2 card!
Even if it was possible to emulate GBA in the DS slot, it would be stupid and unnecessary because the the DS natively supports GBA games in the......wait for it......GBA slot.
You bought a slot1 card to pirate all your DS games instead of buying them, the least you could do is buy a slot2 card to pirate all the GBA games instead of buying them. Common freakin' sense people, come on!
I'm surprised that more people don't aggressively and vehemently flame people that ask this question. They truly deserve to be flamed, if anyone does.