Hi,
I am interested same thing.
Following is just my memo.
Weasis is build by using OSGi bundles(that are calling weasis-framework module) and contains many component maven modules of each OSGi bundles.
You can see weasis-launcher module in your IDEs(e.g, eclipse that is recommended in development web page).
Weasis main window is WeasisWin class which is located in org.weasis.base.ui module of weasis-base module.
You can change it and build whole weasis.
Another way is to use plugin(or "bundle" more correctly).
You can not change main window, but you can add something feature tools(e.g, toolbar item).
you can use "archetype" module in weasis-framework modules that is weasis plugin maven archetype.
Some samples included in this module.
(But, check settings in pom.xml e.g, something version number and so on)
More detail about plugin, please check Weasis development web docs.
Dear, Mr.(Dr.) Nicolas,
I"m glad that if you add more suitable descriptions...
Tatsuaki