Tres Leche Without Homemade Cake?
Tres leches cake is always of the sponge variety.

It is much higher in eggs and the extra protein lets it hold up to sitting in a puddle of milk overnight. A regular white cake would not be a good plan. It would be at best pudding in the morning.

If you are trying to get rid of boxed cake and want it to be a sponge cake you could try beating egg whites and folding them into your batter. 

Personally I think sponge cakes are so easy that I would rather just make the tres leches from scratch and save the boxed cake for a trifle or to give away to someone who doesn't care.