You should use the Qt SDK 1.1.x with its Symbian^3 based target if so neded, if you intend to develop Qt applications. The Symbian "platform" SDKs are for advanced users, they respond to special needs (use of more powerfull Symbian APIs) but have special requirements, including knowledge of toolchain and toolchain evolution (change from abld to sbs for example).
However, why do you need abld in order to integrate a widget in a SIS file? Presumably you have the Qt application built already and have also a valid reason to add an additional file to the package (I can't really imagine why, but that's your problem). So, a like added to the *.pro file will solve your problem:
Call qmake on the *.pro file and the result will be that you get a *_template.pkg file which has the desired addition:
DEPLOYMENT.pkg_postrules += "\"path\\to\\widget\\widget.wgz\" - \"path\\where\\file\\is\\copied\\widget.wgz\""
Now a call to makesis on that *_template.pkg or calling build/run from the IDE will generage the SIS file with the intended content.
"path\to\widget\widget.wgz" - "path\where\file\is\copied\widget.wgz"