How do I convert the excel files into spreadsheet files without changing the existing excel files type in google drive folder?

Question

In a google sheet, I managed to filter file name and file type and list all the particular files needed from a google drive folder based on the conditions into a spreadsheet. I am having an issue to convert the excel files into spreadsheet because I'm opening the selected files using "SpreadsheetApp.openById(id)" in another function() to get the data range from the specific files.

Based on the requirement set, I was been asked to convert the excel files into spreadsheet before getting the range data and export those data into another sheet.

Below are the codes that i created to list the file name and file type, so i believe that the file conversion method might need to be implemented here as well because it involves the file filtering process and these are the files need to be converted later on. Please correct me if I am wrong.

function getChildFiles(parentName, parent, sheet) {

var fileIter = parent.searchFiles("title contains 'completed'and title contains 'Verification 
Visit Direct Suppliers'and mimeType='application/vnd.openxmlformats- 
officedocument.spreadsheetml.sheet'"); 

var output = [];

var now = new Date();
var date = new Date();
var copy = new Date(date);

while (fileIter.hasNext()) { 
var file = fileIter.next();

var fileName = file.getName();

var fileType = file.getMimeType();

var filecreatedate = file.getDateCreated();

var fileUpdate = file.getLastUpdated()

var path = parentName + ' |--> ' + fileName;

var fileID = file.getId();

var Url = 'https://drive.google.com/open?id=' + fileID;

output.push([fileID, fileName, path, Url,filecreatedate,fileUpdate,copy]);
Logger.log(fileType);}

if (output.length) {

var last_row = sheet.getLastRow();

sheet.getRange(last_row + 1, 1, output.length, 7).setValues(output);

SpreadsheetApp.flush();}

var childFolders = parent.getFolders();

while (childFolders.hasNext()) { 

var childFolder = childFolders.next();

var childFolderName = childFolder.getName();

getChildFiles( parentName + ' |--> ' + childFolderName, childFolder, sheet);}}

So I'm trying to implement the file conversion method from excel file to spreadsheet, so that i can extract data from the excel file after the file successfully converted to spreadsheet.