YES.

You for convenience you can use any of the 10 custom fields that are currently available.
However, you can use any arbitrary merge tag you wish as long as it is placed between the two "Curly Brackets." ie. {{your-merge-tag-name}}
*NOTE - This function is mostly used when you are sending mail from your CRM.
AND, you must have data from your data file and or from a custom field in your CRM.

Since It’s not a part of the baseline variables, you can make in anything you want.

Here is a simple example:
Say you have something random like “Loan Date.”
Just write the variable tag like this.
I noticed that you have a loan date expiring on: {{loan-date}}

It's best to keep it simple and not introduce symbols or spaces.

If you'd like to learn more about custom merge tags please contact help@printgenie.io