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: email@example.com and firstname.lastname@example.org
You then create a third email account (email@example.com) that you don’t tell anyone about and your configure both askmeaquestion and news to forward every email to firstname.lastname@example.org
You then configure Postie to check the email@example.com 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 firstname.lastname@example.org emails to email@example.com and firstname.lastname@example.org will all show up in email@example.com and you can is the Category Per User AddOn to assign different categories.