I don't want to invest in a leather hole punch, so how could I add an extra hole (or some other solution)?
Use a power drill with a small drill bit the size of the hole you want. Put a piece of scrap wood under the belt first. After the drill bit penetrates the belt, move the drill up and down to clear out the chaff from the edges of the hole.
(if you don't own a drill, you could go to your local home depot - they often have test drills...be discrete....or...somewhere near you a neighbor probably has one)
I used to use a heated nail. Simply, hold it with pliers, and heat its tip up for a minute or so on the stove fire. Keep your belt stretched firmly (you can hold one side of the belt under you foot, and the other side with one of your hand, while the other hand is holding the pliers) and point the nail to the spot you want to make a hole, and push into the belt. It makes a small regular hole very easily.
I see many sources on the web mention the nail with a hammer. No need for a hammer, heated nail held firmly with pliers, can make the hole you want.
Careful usage of a sharp pointed knife can easily cut you another hole in your leather belt.
Mark out where you want the new hole, and put the knife at the marker and rotate gently. Soon you'll have your extra hole.
In fact most thingies with a sharp point can be used if you are in a real pinch. I.e. one end of scissors, kitchen knives, screw drivers, or even a key if you are a little persistent! Of course the sharper the better...
For many years, I've used the awl point found on many multitools (vis. Leatherman) and multiblade pocket knives. I just use pressure on the belt's metal tongue to make a mark, then take off the belt and drill through with the awl point by rotating back and forth. Once it pierces through, a few additional rotation strokes will enlarge the hole enough for the tongue to push through readily, and you're good to go.
I generally use the pointed end of scissors for punching a hole into my belt.
Once, it manages to pierce to the other end, start rotating it, while simultaneously drilling in; until the hole of desired size(or diameter) is punched in.
If the scissors isn't sharp enough, then using the hack suggested by @Hawk