There are a number of ways that you can use auto-enter field options to copy text--possibly from a related table of standard text entries, into a text field so that it can be further edited. But merge fields embedded in this text will not be automatically replaced with data from your database. Merge fields only work to combine layout text with data from the fields and this won't produce text you can edit.
Option 1: If you use an auto-enter calculation to enter the original language that may then need further editing, the calcualtion can combine data, such as a name with the "standard text" copied into the field.
Option 2: You can use field options to enter the data with merge field like "place holders such as <<FirstName>>, then use a calculation field that subsitutes data from your database in place of <<FirstName>> to produce the correctly merged data.
Here's a demo file that illustrates using the second method: https://www.dropbox.com/s/6xw8buafjuohncu/MessageTemplateBuilder.fp7