Untested, but you should just be able to use ByteArrayInputStream instead.
def excelBytesToDataSet(bytesIn, hasHeaders = False):
import org.apache.poi.ss.usermodel.WorkbookFactory as WorkbookFactory
import org.apache.poi.ss.usermodel.DateUtil as DateUtil
from java.io import ByteArrayInputStream
fileStream = ByteArrayInputStream(bytesIn)
...