When I feed a Christmas cake (home made, but it shouldn't make much difference) I simply prick holes all over it with a thin skewer and drizzle the alcohol over the top. A few days later I invert and do the same on the bottom. You can repeat a couple of times, 2-3 tablespoons at a time, maximum.
For this I wrap in clingfilm, and store at room temperature in an airtight tin or plastic container. This works well for the dark, heavy fruit cakes we have as Christmas cakes in the UK (also traditionally used as the bottom tier of wedding cakes). These cakes keep indefinitely at room temperature so long as they don't dry out. A lighter fruit cake may end up more soggy than moist.
If it's extra flavour you're after, you could steep some appropriate spices in your rum first: a cinnamon stick, a bit of root ginger and a couple of cloves. Big pieces rather than ground, so you can pour the rum off them easily, and do this with a small amount so you can taste it and dilute if necessary.