Works as :custom keyword, but instead of a value takes update function or the second (the first is the original one) argument to the default updater

(use-package use-package-custom-update
  :quelpa
  (use-package-custom-update :repo "a13/use-package-custom-update" :fetcher github :version original))

If you want to add elements to a list testlist (similar to add-to-list)

(use-package emacs
  :custom-update
  (testlist '(1 2)))

By default a variable is modified by use-package-custom-update-union function, which takes a variable value (or its symbol, if use-package-custom-update-updater-use-symbol is set to true) as the first argument, and the supplied updater second, you can customize it by setting use-package-custom-update-default-updater function.

(use-package emacs
  :custom-update
  (testlist (lambda (old-value)
              (append old-value '(1 2 3)))))