Well, there's no such feature in PHPList per se.
But what you can do is add a new PHPList attribute called "salutation".
New subscribers will fill it up in the registration form.
For old subscribers, export the list to a spreadsheet, and add Mr. or Mrs. in each row*.
Finally, import the spreadsheet back to PHPList.
* An easier way to accomplish it is add a temporary column and write just 1 in it if the person is, for example, male.
Then in the real column do something like:
if(d2="1", "Mr.", "Mrs.")
Before you save it as CSV make sure to convert the real column from formulas into values, and delete the temporary column.