True Danish flavoured schnapps should not contain more than 15 grams sugar per liter.
But of course, you can add as much sugar or other sweeteners as you want to suit your own taste.
Also, if your infusion is too weak-flavoured you can certainly enhance the flavours by adding a little, little bit of sugar.
Sugar is a great flavour enhancer.
You can add the sugar directly, but because sugar is more soluble in water than in alcohol, it's usually better to make a simple sugar syrup...
...and add it to your infusion little by little.
Never use artificial sweeteners - NEVER! You will ruin the taste.
White sugar does not impart a distinctive taste of its own. That's why it's commonly used as flavour enhancer.
However, you can substitute white sugar for other sweeteners such as fruit sugars, brown sugar, cane sugar, a mix of different sugars, corn syrup, or honey.
These substitutes are interesting alternatives, if you wish to experiment.
Just remember that the kind and amount of sweetener you choose will affect both taste and flavours.
Another option is glucose syrup. It's not as sweet as the sugar syrup.