Add *User defined* Immunization Expiration Date as an item for letter merge

This would save time when sending Reminders to families. I personally use this feature to send the letters to remind families that their child's immunization form will expire soon but the problem I have is that I have to filter children by immunization expires date (equal day) and send emails just to the families with that specific day. Also, I have to write manually the expiration date. I would like to send only one email and leave the information of the immunization expiration date to the merge letter feature.