- Open "build/Qt/build.pro" project
- Right-click on SomeLib project
- Select 'Add new...'
- Select 'Qt Designer Form Class'
- Select 'Browse'
- Point it to "../../src/" (<project>/externals/SomeLib/src/)
"Add to project" does not contain the project. Pressing Finish causes files to be created and the designer opened, but they are not added to the project.