There are a number of unofficial additional pregens available, but none of them are sanctioned for use in PFS play.
As the various "We Be Goblins" scenarios released so far are all written for Pathfinder 1E, you can't use Fumbus (a PF2 character). Nor can players create their own custom goblin characters, as goblin is not a legal race (except for the small number of goblin characters created using the very rare goblin race boon).
We often run these scenarios at our conventions as an introduction to PFS play. If more than four characters are needed then we simply have more than one player using copies of the same pregen (often with a name change to avoid confusion at the table).
(For non-PFS play, of course, you're allowed to do whatever you like)