@Gayathri, I am not sure about your complete requirement. But, the MergeContent processor is used only when you have to merge the file contents which you have split up-steam using SplitContent processor.
I can suggest you with some workarounds,
1. You may have a common filename for files which follow specific schema
2. Write the file into a local file system using PutFile or into Hadoop using PutHDFS processors
3. Select 'Conflict Resolution Strategy' property to 'Append' when you write the file to disk
Hope this helps.