Can liqueurs be reduced?
Many of the aromatic qualities of liqueurs are due to volatile substances that will likely evaporate while reducing the liqueur. So it really depends on how volatile the substances are that you want to keep.

I performed a simple experiment with Frangelico. So, what I did is (over low heat for 20 minutes) I reduced 1/4 cup of store bought Frangelico to cca. 1/8 cup hazelnut and caramel flavored sugar water/syrup. It tastes pleasant, but the hazelnut aroma is a bit weaker - which might suggest that some of the hazelnut aroma evaporated together with the alcohol and water (wouldn't be surprising) or that the alcohol helped with distributing (hence perceiving) the aroma better or that the sweetness and other flavors now overpower it.

Anyway, as stated above in many cases the aromatic substances in mixtures are aromatic because they are highly volatile - we can smell them cause they escape from the mixture.

So, what can you do? Some techniques that I have used in the past come to mind:


Distillation. Where you separate aromatic substances depending on their boiling points and catch them after condensation. However, the heat might not be good for some substances.
Rotary evaporation. Which became a "hot" technique in molecular cooking =) Here the idea is similar to distillation, but the method is gentler in terms of temperature as it operates under reduced pressure - however it requires special equipment - a rotary evaporator. I successfully used it to separate the aroma from walnut liqueur and to make whiskey syrup.
Freeze distillation. It has been suggested for separating menthol and other stuff from the alcohol medium (you might find suggestions for similar techniques by searching the internet for "homemade [whatever] essence/extract").
Making your own liqueurs/alcohol extracts - with more concentrated aroma than the commercial ones - so you have less volume for the same amount of the aromatic substance. This might be possible as the amount of alcohol in the solution might be able to accept more of the aromatic substance than there is in the commercial liqueur. However, it is very likely they are adding a concentrated/distilled essence, which gives a more clean and stronger flavor than what you can get from  just extracting it (from say hazelnuts) with alcohol.


However, as perviously suggested by @TFD you can just want to avoid all the hassle and buy commercially available essences/extracts or use some alternative flavorings as @razumny suggests. Or simply try the reduction technique if you don't mind loosing some of the volatile substances - and if the result is satisfying enough.

That said, I made very tasty reductions of some liqueurs with very strong flavors (homemade wild blueberry liqueur and Terrano liqueur) to pour over deserts, where I mostly just wanted to get rid of the alcohol taste and to thicken it a bit. So, the results will vary from liqueur to liqueur and your best bet would be to just try it out with small quantities to see how it goes.