You can’t. Cron expressions used to trigger rules are fixed and hard coded.
For OH 2.5 see one of the many Alarm Clock examples posted here to the forum.
For OH 3, create a DateTime Item, configure the card per OH 3 Examples: How to boot strap the state of an Item. When that Item changes trigger a rule to set a Timer to do the actions at the time chosen.
An additional way to handle stuff like this includes the Time of Day Design Pattern. There is even a couple of versions of that you can just download and use, configuring using Items. You don’t even have to write or edit any code.