import           Control.Monad.Except (ExceptT(..), throwError)

data Error = MyError | ThirdPartyError String

-- some third-party function, that doesn't return our type
doSomethingWithX :: Integer -> Either String ()
doSomethingWithX = undefined

computeX :: (Monad m) => ExceptT Error m Integer
computeX = undefined

doStuff :: (Monad m) => ExceptT Error m ()
doStuff = do
  x <- computeX
  lift3rdParty $ doSomethingWithX x

lift3rdParty :: (Monad m) => Either String t -> ExceptT Error m t
lift3rdParty = either (throwError . ThirdPartyError) return