Put a comma before three digit if comma is missing

Put a comma before three digit if comma is missing (as below) in any cell of excel sheet

1000 to 1,000
1000.45 to 1,000.45
2345 to 2,345
5678.98 to 5,678.98

Select all cells, right-click, 'format cells', 'number', tick the 'use 1000 separator' box, press ok.