You can't keep one personally but you can use them in contest halls. Then, you can make pokeblocks. But first you need a Pokeblock case to keep all pokeblocks you make. You would need berries. Lots and lots of berries.
When the ruby and sapphire are recovered the Network machine can be repaired by Celio on One island. The Network Machine allows players to trade with Pokemon Emerald, Sapphire, Ruby, Coliseum, and Pokemon XD.
In Mt Ember.
get both ruby and sapphire to the man next to the machine
in wally's house talk to his dad
In Pokemon Ruby,Sapphire and Emerald after you make a pokeblock, you can check the pokeblock case and you give the desired Pokemon the pokeblock to help it do better in Pokemon contests. After you have received a pokeblock case and you made pokeblock at the berry blending machine go to your key items in your bag and scroll down to your pokeblock case then simply select the pokeblock you want to give your Pokemon.
You can make pokeblocks at any contest hall i would suggest blending berries at lilycove city's contest hall.
The rarest you can get is groudon who is a fire-machine type pokemon.
You go to the topest floor of the Department Store in Lilycove. You get in from the Vending Machine.
Once you deliver the ruby and sapphire to celio the machine will be in operation this means you can go upstairs in any Pokemon center go to the trade center and trade between ruby, sapphire and emerald version.
Pokemon ruby's version IS Pokemon ruby.
fix the machine on island 1 find the ruby and the sapphire stones
After fixing the Network Machine on One Island, you can trade a Mudkip from Pokemon Ruby or Sapphire. You can't catch a Mudkip in Pokemon FireRed.