If you're only short by 5 mm (2.5 mm per side), all you have to do is pull your dropouts a little farther apart as you're mounting the wheel. This is easily done with the bike upside down. Just set the hub in place, with the axle nuts resting at the slot openings. while positioning yourself behind the bike, use your fingers to pull the stays apart, while using your thumbs to guide the axle into the slots. Once things are lined up, gravity takes over and the wheel fall right into place. You'll find it takes minimal effort to pull your stays far enough apart, to accomplish this.
It's okay to "flex" your stays apart, a bit, since aluminum will flex, and 2.5 mm per side is not enough to cause a permanent bend, which you don't want to do with aluminum. You see that whenever you remove the wheel again, your stays will spring back to their original positions.
If you don't trust this method, then don't do it, but I have 3 (soon to be 5) aluminum Electra's done this way, and 2 of them are 10 mm to narrow. And, if you look around the web, you'll see many other people with aluminum framed bikes that have upgraded to wider geared hubs. I've never heard about a single incident where someone had damaged their frame by doing this.