Hi Francesco, and welcome!
Please see the FAQ: What's a reproducible example (`reprex`) and how do I do one? Using a reprex, complete with representative data will attract quicker and more answers. It is probably not required for this kind of question, although it might help me better understand the goal.
Also, people pretty much stopped counting when the number of R packages crossed 10,000, so for all but the most popular packages, there may be no one familiar with self-organizing maps who sees the question.
To start thinking about this problem, I'd like to see a sample object from the package that produces a cluster object that can be directly inspected, rather than by using a visual plot. That will help me start thinking about whether a vertex cluster in a graph object (graph as in network, not visuals) has an appropriate statistical test.