They already do spawn villager zombies, its just way rarer then 1/6 or 1/7. This would be a custom plugin which in my opinion would be a waste of time/money/effort when there are very simple and quick ways always in place that can give the same solution.
1. Put villager eggs in the /warp shop (50k each, I would not put it any less or you will mess up the eco).
2. Turn on villager breading in the "WILD ONLY" (im not too crazy about this idea, but its there).
There are other options too, but they are a bit more complicated, but still easier and better then a "new plugin".
