You are right that it would be a very nice feature, a nice-to-have and logical to put in a webmail-client. However; the feature isn't available with IMAP; so it is impossible to build. If you want it that hard; it might be a good idea to focus on how to integrate an external application (a password changer) in RoundCube, which would be a nice question and it would be good if there would be an howto on modifying templates, and adding (custom) functionality to RoundCube.
Sjon