I've achieved a similar effect by creating a table and assigning extra rolls to certain outcomes for Random Encounters (see this module
https://www.fantasygrounds.com/forum...tor&highlight=)
What I did was to have the table roll a 1d100, but for events I wanted to be more common (i.e. to make common creatures appear more frequently) I assigned them a larger range. So a roll of 5-9 might give you a goblin encounter, while only a roll of 99 will give you a green dragon encounter. This works, but becomes tricky once you have the table populated and want to adjust values. If you decide you want to give an event a larger probability, say expand the range from 5-9 (5%) to 5-12 (8%), you then need to change the other events that you had previously set to occur in the 10-12 range, which might cause you to end up having to change the whole table. There are ways of mitigating that as well, but hopefully this is of some use to you.