I'd personally recommend LibreOffice Calc as it gives you more control over the character encoding - Excel tends to default to its own encoding, but you can specify UTF-8 in LibreOffice Calc very easily, and AtoM excepts a UTF-8 encoded CSV file.
Whenever I have previously imported a CSV file where I've seen linebreaks inside a cell while looking at it in a spreadsheet application, AtoM has respected those on import - so the application seems to be handling the newlines for me.
Also, there is this in the docs - not sure if it helps:
Cheers,
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