While exporting, you could replace line breaks in cells with (let's say) tabs (or a combiantion of strange chars, like "§#§").
Then, when reimporting, replace the tabs with line breaks again
[EDIT]
Oh, it's from Excel... nice. It's even better, then!!
You can export rows having the cells surronded by "".
By doing so, you know what belongs to the cell and which is the real row terminator.
While importing, just remove the extra quotes.
[EDIT]
In excel, when you export as csv, you can choose to save the cells beding "quoted", so ALL the cells will be quoted.
Like:
"CellA1", "CellA2", "This is a cell with spaces", "CellA4"
"CellB1", "CellB2", "CellB3", "This is a cell
with
carriage returns"
It's automated while exporting if you choose to do so