There are several  ways to use multiple identities with one account.

By manually typing an alternate address in the From field while editing the message

With a Quick Template (QT) You can change the contents of the From header with the %From="something" macro. When you place that macro in a QT you can even assign a hotkey to that QT, so you can easily change to another identity.

With a folder identity: You can assign identities to folders, via the folder properties. This is method I'm using most. Messages to certain addresses are filtered to their own folder and that folder has the same identity set in the properties. As I filter my outgoing mail in the same way that means that my incoming and outgoing mail with the same address is moved to the same folder, so I can use a threaded view for those folders ad have the messages grouped by thread.

With address book templates. When you've got a number of contacts that always get their mail from you with an alternate address, then you can set that address in an address book template, that can be either a personal address book template or a group template. This can be combined easily with the QT  method listed somewhere above as you can call a QT from another template with the %QInclude macro.
When using address book templates you should know that for a new message the used template will be the first template encountered in this order: personal address book template, addres book group template, folder template, account template. Note that as soon as you start typing in the message body the template is set, so first pick the proper addressee when you want to use an address book template.



I hope this will do for you.