I’d add a note of caution: the advice is state-specific. In the ACT the only way to get money out of the sinking fund is to spend it in accordance with the sinking fund plan, which can be amended by ordinary resolution so not that hard to do just before resolving to spend. Still it has to be on one of the things that a sinking fund can be used for. You can not transfer to the admin fund. Transfers are a one-way street here.
However, you can as suggested below. The AGM can resolve to levy more for the admin fund and less for the sinking fund which amounts to the same thing. We just did the same thing the other way around since we had a surplus from last year in the admin fund but had a new expense to cover in the sinking fund.