OKay, so one day, my friend got a Fire/Electric type Magneton in Blue. He told me that he did it like that:
1- Catch a Magnemite/Magneton in R/B (Not sure about Yellow)
2- Trade it to G/S (Not sure if Crystal works too)
3- Trade it back to R/B. It should be Fire/Electric type.
Note that I'm not sure if it works all the time. The reason I don't know if it will work with Yellow and Crystal is that my friend used Blue and Silver for it.
I think that Magnemite turns to Fire type since Magnemite gets Steel type in G/S, and when it gets traded back to R/B it replaces it with Fire (I'm not sure if it could take another type.
In G/S, Magnemite gets an hex number that gives it the Steel type, and when you trade it to R/B, the hex number doesn't gives any type anymore, since Steel type doesn't exists.
Even if it gives no type, the game "thinks" there should be a type in that emplacement, and it gives off the type that is the first in the hex list, Fire.
This could be useful, since by gaining Fire Type, it gains a resistance against Grass type and Fire type.
I just need someone to try it to see if it works, prefferably using Blue and Silver, since these were the games my friend used. If that doesn't works, then I could just delete the thread or mods could lock it.