As I said, you could. You do so by selecting the line, then choosing Insert>Table>Convert Text to Table. Your line will become a nested table. Unless you've set the internal cell margins to 0 left & right, though, you're liable to find you new table has its own left & right borders displayed within the existing cell. And, even with that setting, you'll probably need to delete the new table's left & right borders so they don't make the outer table's border appear thicker at that point. Splitting the cell vertically and moving the components about requires less circumlocution.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]