Hello,
 
I have posted an algorithm (multiclass.m) which makes it possible to do multiclass classification using several two-class classifiers.  
 
There are several possibilities for such classification, such as one-versus-all, all-pairs, and Hamming coding matrix. These can be chosen using the function parameters. Note that this function cannot be run using the classifier GUI, only the command-line interface.
 
I have also uploaded a demo data file, multiclass_xor.m.
 
Elad