How can I incorporate alcohol (rum, in this case) into an existing cake recipe (pumpkin cake, for example)? Can I simply replace the water with rum?
You can also try, rather than putting it in during the baking process, to pour the rum into a pan big enough to fit te cake or loaf and have it soak up the alcohol. Additionally, rum is a very strong flavour, and might overpower or conflict with the pumpkin, so be careful not to use too much or you might end up with pumpkin flavoured rum cake as opposed ot rum hinted pumpkin cake.
If I just want to add aroma to the cake, I just add a small amount to the batter. If I have an ingredient like dried fruit or raisins in the cake, I might soak them in rum for a couple of hours before incorporating into the batter. If I want an actual boozy flavor, pouring a modest amount over a baked cake, while still in the pan, will allow it to absorb more. Finally, you can make an icing or glaze from a mix of powdered (confectioner's) sugar and liquor instead of water.