Hi Gordon,
yes, it is possible. The most obvious way is to make your own gzipall.bat file, including whatever Javascript you want, and excluding whatever you don't want.
Some words of cautions regarding ExtJs are perhaps appropriate here. This is not meant to turn you off, but merely to perhaps raise some issues you may not have thought of. Choosing a client-side-framework is a big deal, and has implications down the road.
a) Licensing is the biggest hurdle, and one of the primary reasons we didn't choose ExtJs for NetTalk 5. In short you either need to purchase a commercial license from them, or release your app under the GPL license. While it's easy to ignore this in the short term, the longer-term ramifications can be substantial.
b) the NetTalk templates are obviously optimized to make use of the jQuery framework. I wish I could just add support for ExtJs as well, but there just aren't the resources to currently do that. I know Patrick de Laet started a project to develop NetTalk templates for ExtJs, and Pratik Patel I think has done some work on that as well - but in the long run you may want to consider the long-term gains and losses from stepping away from the mid-stream. Ultimately, it's _templates_ that provide the productivity gains we enjoy, not the framework. Moving from a template driven system to a hand-coded system does have implications for long-term maintainability and extendability.
c) In chatting to users I've noticed that many do like the "windows program" style ExtJs delivers. Be aware that this is a _css_ issue though, not a _framework_ issue. If you want to make a jQuery app look like Windows (which frankly, personally, I don't) that's simply a case of getting the Css right.
As I say, the above considerations are not meant to scare you away - just to cover some bases you may not have thought of. If there is, for example, functionality in ExtJs you miss in NetTalk then please, by all means, post here so I can give some indication of the prospects of that functionality in NetTalk. From a "future direction" point of view it's also a good idea to have a look at the jQueryUI roadmap. (
http://wiki.jqueryui.com/w/page/12137947/FrontPage). While we're not limited to those UI elements (we're also incorporating some plugins, and writing our own plugins), we'll be supporting all the jQuery UI widgets as they become available.
Cheers
Bruce