Save custom page size (not template)

Is there a way to save a custom page size? By this I don't mean a template for new scores but rather a page size available for existing scores. This way any score could instantly be converted to the custom page size.

(My understanding is that templates are only available for new scores; existing scores can't be adjusted to conform to the custom template.)

If saving a custom page size isn't yet available, can anyone point me to the location of predefined page sizes in the source code?

Many thanks for any assistance.