Like a 6x weakness?
Technically yes. If a Pokemon has a x4 weakness to Fire and has the Dry Skin ability, they will end up having a x5 weakness to Fire.
As of now, the only Pokemon who actually have this are Paras and Parasect.
In addition to the Paras with Dry Skin senario, a Pokemon with a 4x weakness to a weakness of Grass under the influence of Forest Curse will then have an 8x weakness to that type.
E.g Malamar - Dark/Psychic with Forest Curse = Dark/Psychic/Grass = 8x weakness vs Bug
Dragonite - Dragon/Flying with Forest Curse = Dragon/Flying/Grass = 8x weakness vs Ice