If you select part of a column or row, then paste the copied cells to:
• another location in the table, where there are sufficient cells (empty or not) to accommodate the copied data, the data will be pasted into the existing column or row;
• another location in the table, where there are insufficient cells (empty or not) to accommodate the copied data, the paste will add as many new columns or rows as are required to accommodate the pasted data; or
• an end-of-row marker, or immediately before the first paragraph following the table, a new set of cells will be added to the table.

If you copy an entire column or row, then paste the copied cells to:
• the start of another column or row in the table, the data should paste as a new column or row;
• somewhere in the table other than the start of another column or row, the paste will add as many new columns or rows as are required to accommodate the pasted data; or
• an end-of-row marker, or immediately before the first paragraph following the table, a new set of cells will be added to the table.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]