Short answer: You don’t.
Even if it can save as Excel I wouldn’t expect it to work properly unless resaved in Excel.
You can try copy paste but I’m not sure how well that will work, either.
You can import a .xls file.
First, go to Preferences > File handling > check "Create links ... spreadsheet files" in order to be able to update your table when modified in Excel.
Note that you won't be able to get any formatting from Excel file, so you should create a custom table style, along with paragraph and character styles.
While importing your Excel file, check "import options", select "Unformatted Table", and apply your table style.
Of course, no dynamic math operation can be make within InDesign Table, you must do it in your source file, then resave it and update your link.
If you properly used cells, paragraphs and characters styles, the formatting will be preserved when updating link to the spreadsheet.
Don't forget your .xls file is now linked to your indesign file (just like a picture), so don't move or delete it without caution...
I'd agree with Bob: Even if you save as an XLS file from Numbers, I think it must be saved in Excel.
I use PC, so... I didn't know Numbers...
So I misunderstood the question... I just understood "spreadsheet file using numbers", which is quite different...
Numbers is the spreadsheet in Apple’s iWork suite. And Excel files are NOT linked by default in any recent version of InDesign.
Yes, I got it now!
I know Excel files aren't linked by default, but it's easy to change in the preferences...
To come back to the initial question, you said you can save from Numbers to XLS...
As you say, problems might come up without resaving in Excel... or not... I don't know, but I would give it a try.