How many different fax numbers are there? Can the fax server read the number from the subject line of the mail?
I think you would have to have an email account for every fax number that the fax server services if you want to route messages that way.
Currently in alert central, email addresses are fixed, and must be assigned to a user. There is no mechanism to synthesize addresses based on a property of an alert.
While it would be possible to "capture" the fax number from the subject line of the email and assign it to the message, object, or severity fields of the alert, the only thing you can do with it after that is use it to route to different groups.
So, unless you have a finite number of fax numbers to send to, I don't think you will be able to achieve this use case with alert central.
If you do have a smaller set of numbers you are working with, you could configure a group/user/email address per fax number, and use the group routing rules of the email alert source to match the captured number to the correct group for that fax number.
I hope that helps.