It is only possible to have one "default" reply to email address.
Within each Task, you can either use the default reply to email address or manually insert a custom email address.
Each Task can only have a single reply to email address.
The reply to email address however is not the same as the "email account from which nprinting is sending it's emails". That account is the SMTP server account, and there can only be one of these. Recipients will not see this email address, it's just the way NPrinting connects to the SMTP server.