Hi Vivek,
This can be done you can add customer specific fields to the leave request application for this you have to add the field to the table VV_PT_FLDSEL_NAM.
Then assign the field to the leave type in the table V_T554S_WEB then this would appear in the leave request application.Then do the changes to the web dynpro java application where you can make this field as selection field where in the drop down you may have full day or half day.
As per the standard there is no selection of half day or full day as drop down.You are in EHP6 why you are using the Webdynpro Java application you can use the Webdynpro ABAP application instead.
Regards,
Mithun K