As I have had the same problem (I need to use libqjson while using MADDE/QtCreator), here is how I solved it (ugly hacks, but well, if there is no official way…):

• Install libqjson on your device (N900 in my case)
• In order to type less in the following commands, set a variable with the path to the sysroot:
  
  Code:

  export SYSR=~/.madde/0.6.72/sysroots/fremantle-arm-sysroot-10.2010.19-1-slim

• Copy the development headers from your system (or from the device, but then you need to install libqjson-dev there, too) to the sysroot:
  
  Code:

  cp -r /usr/include/qjson ${SYSR}/usr/include

• Copy the library itself from the device (to ensure that you got the very same version):
  
  Code:

  scp n900:/usr/lib/libqjson* ${SYSR}/usr/lib

• This step is important, otherwise MADDE will just fail when building the debian package, stating "dh_shlibdeps failed": Copy the dpkg meta files from the device:
  
  Code:

  scp n900:/var/lib/dpkg/info/libqjson.* ${SYSR}/var/lib/dpkg/info/

Then, in the Qt project file, add: