I can't confirm that ConvertExcelToCSVProcessor processor does not work with xlsx.
It works, only thing I haven't achieved until now is to format the csv-output in the right manner.
Maybe this will help you to get it?
My test xlsx data:
id title first last 1 miss marlene shaw 2 ms letitia jordan 3 mr todd graham 4 mr seth martinez 5 mr guy mckinney 6 ms anna smith 7 mr johnny johnson 8 mrs robin white 9 miss allison williams 10 mrs erika king
My not correct formatet temporary csv-output: