Can "Subscriptions" plugin be customized to send all posts once a day

This would be so great!!!

Although site owners can’t enable this option, your subscribers can choose to receive daily, or even weekly digests, in their subscription settings.

We’ve been thinking about adding an option to allow site owners to set a default themselves, though. You can follow our progress on this here:
https://github.com/Automattic/jetpack/issues/785

Thanks for the feedback!