No it is impossible.but you can get the starters
First of all that guy is lying... put ur ds on Thursday and play the radio station on the top right of the radio and walk around... unfortunately, every battle you have to play the radio station. i just got a shinx and fully evolved it into a luxray.
Other Person...You can't actually go to the Sinnoh region in HG/SS and battle gym leaders and such, but you can get some Pokemon from there by playing the Sinnoh tunes. Both of these people are sorta right, since you can get the starters by trading since the games are all from the same generation.
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well heartgold and soulsilver takes place in the kanto and johto regions, so obviously the pokemon from those regions are there, though probably not in the same place as the original games. there is also hoenn region and sinnoh region pokemon in this game, but in order to get those you have to listen to a certain type of musice on the radio on a certain day in the game. for more help on where to find hoenn and sinnoh region pokemon in these games, you should really go to seribii.net
In Pokemon HeartGold (And Pokemon SoulSilver) The bike road is on Route 17 in the Kanto region.
Cynthia is not the champion in the johto region in Pokemon heartgold,you can't find her in it, but she is the champion in Pokemon pearl,diamond, and platinum versin and that is the sinnoh region.
you cant get to the sinnoh region in Pokemon hart gold or soul silver but you can get the starter Pokemon at slip co (i think its called) once you done all gyms and defeated red (the Pokemon with emerald version).
It is only in HeartGold & SoulSilver and Gold & Silver you can go to different regions :/(and Crystal)