I see... well if the action does not exist in the word interface, your option is probably to create a new action that does what you want.
So, I see that the Add Row action has code that looks like:
Dim doc as Object = GetDocument(handle,documentname)
doc.Tables.Item(TableNumber).Rows.Add()
I would suggest that a new Add Column action would be extreamly similar and probably look something like this:
Dim doc as Object = GetDocument(handle,documentname)
doc.Tables.Item(TableNumber).Columns.Add()
NOTE: I don't know if that is the code you need, that is my non-research guess. I would suggest using google to find out for sure. This kind of thing:
https://msdn.microsoft.com/en-us/library/office/ff837258.aspx
However, I would guess that adding columns is more likely to impact the general formatting of a word document (such as going over the right of page boundary) - so even more actions/logic would probably be required to somehoe counter that.