Can I get all Hoenn and Kanto starters in Pokemon SoulSilver?

If you have PKMN LeafGrean and PKMN Ruby (or their counter parts), you can use a feature to migrate PKMN up from them. All you need to do is choose a starter, catch 6 more PKMN, then transfer the starter up with 5 of the others (because you need 6 at once to transfer). Then you restart and repeat with the other starters and games.

Because you can get 1 Kanto Starter (by beating all Gyms and Red, then talking to Oak) and 1 Hoenn starter (by getting a PKMN from Oak then going to the PKMN Fan club in Kanto, then a guy named Steven will tell you that Latias or Latios is roaming the game, then you go to Silph Co. and he is there. Talk to him, he asks which stone you want (red, blue, or green) and will give you 1 Hoenn starter with the respective color), you only need to transfer 4 times.

But based on your main question, it is impossible to get all of the Hoenn and Kanto starters in game.