To generate a wild Pokemon battle, call pbWildBattle(X,Y) where X is the species number of the Pokemon, and Y is its level. See PBSpecies for species IDs.
To change the kinds of wild Pokemon found in an area, you can modify the file PBS/encounters.txt (see that section in this page).
After doing so, it is enough to put grass tiles (or other tiles with terrain tag 2) on the map for wild Pokemon to appear there -- no events are necessary.
The file encounters.txt supports two different encounter types: Land and Cave. For Land encounter types, wild Pokemon will appear only on the grass.
For Cave encounter types, wild Pokemon will appear anywhere. Several other encounter types are also supported; see the "PBS/encounters.txt"
section for details.