A handy method was found recently to avoid the above error.
Step 1: Add the user defined xsd file in the following catalog to access it through application server %BPEL_HOME%/bpel/system/xmllib
step 2: import the xsd file in any wsdl file of the project as follows
<import namespace="http://xxx.com/xxx" location="http://your_localhost:8888/orabpel/xmllib/XXXX.xsd"/>
step 3: add the following namespace in your *.bpel process file
Use command line tool to compile the project.