Not really, unless you don't mind users changing the line settings each time. If only a few people will need this feature, you can set-up each person's sip account on a different line in each Express Talk installation. Then the person can select their line (1-6) to make phone calls. This is obviously not an ideal option, but It is the only thing I can suggest at the moment.
If you really want to have this feature, you should be able to create a program that lets users log in, and based on the user, change Express Talk's settings in the registry to reflect the new SIP account settings.