I forward my email from other accounts to one main account which is set up in WLM as the only POP3 account. I set up other accounts as IMAP so those accounts will appear in the drop-down list for sending mail. The remaining inconvenient part is that mail sent from an IMAP account is saved in the Storage Folder area, so I need to manually move it to the POP3 account sent folder.
Alternatively, since message rules can be applied to POP3 accounts, it should be possible to set up rules to move all incoming messages from all POP3 accounts into a customized set of Storage folders. Then when sending mail, use the IMAP trick so that all the sent mail will be in the Storage area also.