You can map a CAS number to a text field in the in the external enumerator, but it will not load the structure data. If you wish to add structures using CAS numbers to an external enumerator it can be done manually using the 'Quick Add' feature.
Solution:
1.Navigate to the External Enumerator
2.Click 'Add Reactant'/'Add Product'
3.In the 'Chemical Structure' select 'Fixed Value'
4.Click 'show drawing'
5.In the 'Quick Add' field type the CAS Number
6. Select the number from the dropdown menu
7. Click 'Add'