Zaheer Abbas writes:
I wish to include the recipient's username in the email content, similar to a personalized greeting (e.g., "Dear [username]").
Are you sure you're using the right software? Much as I love Mailman, it is not intended to be a "mail merge" software used for generating personalized replies from template + database. There is software specifically for this purpose, and you might be better off investigating those, since they will have documentation suited to precisely this need.
That said, besides Mark's remarks which you should read carefully, it would be "simple" (for a programmer with some Python and database skills) to write a handler that reads a template file (or accepts a template email) and populates template variables with values from the database. Then insert the handler into the posting pipeline.
If that's attractive to you but you don't have the skills (and aren't in a position to acquire them quickly), there are consultants on the job boards on the Mailman wiki and Python site. It doesn't take much specialized knowledge of Mailman.
Steve