This will work with standard C# string+integer concatenation, so
System.Random randomNumberGenerator = new System.Random();
...
int randomSkinNumber = randomNumberGenerator.Next(1, 61); // creates a number between 1 and 60
string skinName = "footballthug_" + randomSkinNumber; // or if you prefer: = String.Concat("footballthug_", randomSkinNumber);
SkelAnim.skeleton.SetSkin(skinName );
In general I would advise to search the web for general C# programming questions, this will yield results faster.