Why a Custom processor?
Could you not convert the Excel file to CSV and use the ValidateCSV processor?
If you do want to make a custom processor, then I would suggest that you configure it to read in the json from a file path as a property based on the excel it is transforming.
So you would have your processor read in the file 'MyExcel.xls', then pick up the appropriate JSON by looking for a file called MyExcel.json using the attribute ${filename}.json, the do the validation.
There are many examples of processors which read a particular file from HDFS or the local filesystem based on an attribute, so perhaps you can start there.