Well , after a few hours of trying the problem is solved. I didn't set up correctly in Excel. I had a list of two columns of names which I want to combine with 'UndirectedEdges' In Excel I already concatened the strings and imported them in Mathematica. But what I overlooked is that Mathematica sees the concatenation as one string and not as two separate string. So I splitted the names over two colums and now the import works as expected.

Thank you for your time, Chiel Geeraert

Just writing helps a lot ! I think I overlooked the problem. With the import from Excel quotes are added , so instead of {{ John, Peter}} I get {{"John,Peter"}}. I have to get rid of the quotes ? Any idea ? Thank you