Hi, just want to update my status on trying to add combobox in the wxSmith. 
I have start to write the code for wxComboBox , the XML loading code. I refer to other widgets's code to add in appropriate code in defwidgets.cpp/h, creating combobox.cpp/h . The most difficult part  is on the combobox selection . 
  I have refer the help file bundled with wxWidgets 2.6.1 and found that wxComboxBox have 2 constructors , the main different is one of it use wxString choices[] and other use wxArrayString as selection. 
  I have decided to use the wxArrayString approach since it looks easier. After coding an special function to load selection to wxArrayString, and get ready to have a final compilation, the code could not compiled !!  Finally , I found out , in wxwidgets-2.4.2 , only have 1 constructor for wxcombo , I am refering to the latest documentation (2.6.1) to code , hahaha .. . 
  Therefore , now is in the status to recode all in wxString choices [] 
 :cry: 
  One thing need to mention is , it seems like there is no engine (code) to allow the wxwidgets's properties to be edited by poping up another dialog , I believe in wxcombo's selection we need that. And in fact ,  a lot of cases we need that such as color selection.
  Maybe , it comes to a stage to upgrade the properties engine.