No. Encounters are completely random and there is no way to set an encounter unless you use cheats or something. However, there is something you can use to your advantage. It's called 'dupes clase' which is basically if you encounter a Pokemon or its evolutionary line, you are free to skip it and get a different encounter but only if you already have said evolutionary line
For example, in route 201 in Sinnoh, you can get a Biddoof, Starly and Kricketot. By catching a Bidoof in route 201, you can skip your next encounter on route 202 if it is another Bidooff, allowing you to get a second chance and get either Shinx or Starly in 202
In your case, you just have to go to other routes and capture a Zigzagoonk, Poohycena, Wumrple, Seedot and Susrkit. Since Ralts can be found on route 102, by capturing these mons from other routes, you can skip every encounter until you have a Ralts since the dupes clause states that you can skip an encounter if you already have it or any of its evolutionary line in your box or party