The answer is “you can’t do that directly”
However, what you can do is have each separate inbox forward their mail to a central mail account which you then configure Postie to check.
For example you have 2 email accounts that you want your users to send email to: firstname.lastname@example.org and email@example.com
You then create a third email account (firstname.lastname@example.org) that you don’t tell anyone about and your configure both askmeaquestion and news to forward every email to email@example.com
You then configure Postie to check the firstname.lastname@example.org email account.
Many people have this arrangement so that emails sent to each account get assigned different categories. To accomplish this you need to purchase the Category Per User AddOn and follow the directions for configuring Multiple Inboxes.
If you are using Gmail you have a simpler option. With Gmail it will ignore anything after the plus sign (+) so if your email is email@example.com emails to firstname.lastname@example.org and email@example.com will all show up in firstname.lastname@example.org and you can is the Category Per User AddOn to assign different categories.