In the G4MaterialPropertiesTable::AddConstProperty method of G4MaterialPropertiesTable (located in G4MaterialsPropertiesTable.icc), it is impossible to add a new constant property key because the first call is to GetConstPropertyIndex on the provided (NEW) key, which doesn't exist and throws and exception. 

I believe that it should do something like below: (unless of course it's not intended to add new keys ... ). This would make the mapping of constant parameter names non-constant. Another option would be to change the method name to SetConstProperty since really, as it stands, the method can set only existing constant property key values. 

The reason I think it should allow for additional constant property keys is because the user may want to register newly named properties for different materials.

inline void G4MaterialPropertiesTable::AddConstProperty(const char *key, 
  G4double PropertyValue)
{
  // Provides a way of adding a constant property to the Material Properties
  // Table given a key
  auto it = G4MaterialConstPropertyName.find(G4String(key));
  if (it == G4MeterialConstPropertyName.end()) {
    G4MaterialConstPropertyName.insert(G4String(key));
  }
  G4MCPindex index = GetConstPropertyIndex(G4String(key));
  MCP[index] = PropertyValue;
}