Wild Pokemon Method
Yes you can. Go to Celestial Tower, with lot of Ultra Balls and equipment that can catch around 20 Elgyem. Catch 20 Elgyem. Go to the Center. See their ability. Release every non-Synchronize Elgyem. See the nature of the Synchronize Elgyem. Is it Modest, for example ?
If yes, so Use that Elgyem as the leader and go somewhere. For example, Virbank Complex. Catch the 2 first Magneti you see, they must be modest.
This may take time, but this help me searching for the best Growlithe or Riolu.
Though, Synchronize has a lot of chance of finding a Pokemon from the same nature as the Synchronizer if the Synchronizer is the lead.
A parent is Jolly and you desire his child to be Jolly too ? Simple as "bonjour" (hello):
Give the Parent who has the desired nature an Everstone then Hatch the Egg. It will be of the same nature, except if the second parent also hold an Everstone. If this case occurs, then the Baby has 50 % of chance to be Jolly, 50 % to be from the second parent's nature.
Hope I helped :D