Unmerge all cells in Numbers xls file
There's no automated way to do this. 

As you would know, when you merge two or more cells the data all ends up in the new merged cell. However, if you unmerge the cell in future, Numbers will retain the data but keeps it in the left most cell (or, in the case of merged cells across rows and columns, data ends up in the top left cell when unmerged).

This behaviour is standard across spreadsheeting software (e.g. MS Excel, etc). Typically, if you want to unmerge the cell and then retain the data in the other cells as well, you would first unmerge the cell and then copy/paste the existing data from the first cell into the others.

If you needed to automate this because you had a large number of merged cells to unmerge and retain the data in all cells, you would need to look at using something like AppleScript to achieve what you want. Of course, this assumes you're talking about Numbers in macOS rather than in iOS.

Finally, your question refers to an Excel file (the .xls file you mention), so another option would be to use Visual Basic in Excel to make the changes first before opening the file in Numbers. However, this too would require coding a custom solution just like you'd have to using AppleScript. Any such option would have to be done within Windows or macOS (i.e. not Excel for iOS or Android).