Chocolate hazelnut: efficacy of mixing crushed&roasted hazelnuts to sauce
The only way to really know for sure is to give it a try, but I suspect that for a liquid application it will be difficult to get the sauce both thin enough to dissolve easily, and smooth enough to avoid a grainy or chunky texture.

You'll have to blend the hazelnuts very well at minimum - I'd pulverize them first in a blender or food processor, add the chocolate sauce, and then blend for several minutes' more time.  You may have to thin the result with a little water (warmed so as not to seize the mixture).  A run through a sieve or cheesecloth to strain out remaining pieces would be a good idea.

With all that blending and filtering, your yields will probably be pretty low.  One alternative you could try would be a hazelnut flavoring of some kind.  A natural extract should lend a distinct hazelnut flavor to any other sauce or mixture (I'd avoid cheaper extracts labelled "artificial" though).  From a practical perspective it will be easier to mix without texture issues, and if purchased in quantity probably won't be much more expensive than preparing hazelnuts from scratch.