How can I change all values of column A (currently empty) to be 5? In Excel, I could do Paste Special > Multiply. Is it possible in Google Docs?
I am confused by your explanation of how you do it in excel (past special then multiply) this will multiply all the values in the column by the value you are trying to paste. In your example you said that column A is empty. That will leave you with a column of zeros.
So here is my approaches: