It would be both cheaper and easier to buy a complete 12x32 cassette. Changing from an 11T small cog to a 12T small cog would require buying both a 12T first position and 13T second position cog as well as the 32T big cog. Individual cogs are hard to find and expensive if you do. Also, to clear a 32T cog, you may need an MTB rear derailleur as most road rear derailleurs won't work with that large a cog.