If the recipe calls for oil, butter, or any other fat, then you can try to scale the other quantities to match it.
If not, there's still a chance you can salvage it. The oil doesn't actually chemically react with the mix at all, it just "coats" it; in fact, most cake recipes use some amount of fat in order to slow gluten formation (i.e. stop the cake from getting tough and chewy).
However, if the recipe called for 1 1/4 cups water and you used the same quantity of oil instead, then that is going to be way too much oil for a single serving.
What I would do is double or maybe even triple the cake mix, make sure it's very well mixed with the oil, then add however much water you needed in the first place.
If you realized your mistake early (i.e. if the original recipe called for a lot more than 1 1/4 cups of water) then you might be able to just proceed as normal and end up with a very moist cake, but more likely you'll still need to add some more cake mix (and the corresponding amount of water) to compensate. I'd still double it, because otherwise you'll be stuck trying to figure out how to use the leftover mix.
Don't just add flour; the amount of water that the recipe calls for is proportional to all of the other ingredients in the mix, and it's virtually impossible for you to know how much extra water you'll need to compensate for the added flour. Besides, you'd only end up with a bland, flavourless cake if you did that.
If none of these options appeal to you then I'm afraid it's destined for the trash, unless you want to prepare your own cake mix from scratch and combine the two recipes.
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.
Best Answer
Short answer - not really. Fat is an essential component in any cake, and milk just isn't very fatty - about 5% for whole milk. You can make cakes with milk, but they require totally different recipes: you can't simply substitute milk for oil.
Bear in mind that you're distributing the cup of oil throughout a whole cake, so that any one slice will only have a fraction of the oil. I assume you'll also be sharing the cake with others, so you'll be 'spreading the calories' somewhat.
You should be using a neutral oil like canola (rapeseed) oil anyway, as it has a relatively mild flavour. A popular alternative these days is to substitute all but a couple of tablespoons of the oil with apple sauce (really), but this can be hit and miss and doesn't work with all recipes - it usually works best with things like quick breads (scones etc).
Of course, there's also the point that cakes are supposed to be a treat, and they'd be less of a treat without the fat, in which case why bother? If you want to be healthy, make a salad. If you want a treat, make a cake and don't worry about the contents too much.