zombie of wolf

Yes it is, there's a line down in "private void SpawnBear(Vector3 pos)"

BaseEntity bear = GameManager.server.CreateEntity("assets/rust.ai/agents/bear/bear.prefab", pos, new Quaternion(), true);​

You'll want to change it to /wolf/wolf.prefab. I don't know if zombies still exist and I'm JUST now getting back into Rust, haven't actually picked back up my coding tools yet, but the change is relatively easy, it's literally just changing bear/bear to wolf/wolf. 

Handling multiple types of spawns at the same time is more difficult but doable, it was discussed when I originally released the mod but I think I decided against it. Bearrels is for bears was the thought process. It's easy enough for someone requesting the feature to edit one line if they really want wolves. You could also clone the file for each type of thing you want to spawn for multispawns, IE one bearrels file, one wolfels, one zombies, etc. It'd be possible for all of them to trigger at the same time though.