I've never done this before, but I just created a table with six columns. I highlighted the whole left column (top to bottom). Then I clicked the "1,2,3" button (start a numbered list). It automatically numbered each column. Then I inserted a column in the middle. Word automatically adjusted all the numbers. Then I put my cursor in row 4 and clicked the increase indent button. The number four automatically changed to "a" and the following numbers automatically readjusted. I think the trick is to just make sure your whole first column is selected and blank when you choose the automatic numbering feature.