Hi,
Firstly, as you will have noticed the RM UI does not support the customisation of repeating properties as of yet.
If you wish to edit the RM custom model directly you will find it in the Data Dictionary area of your repository within a folder called Records Management. The model is called recordsCustomModel.xml and if you take a look inside, assuming you are familiar with the Alfresco content model structure, you should see where to make the change you require.
I would, however, like to stress this type of modification isn't supported or recommended. By all means feel free to experiment and see if this works, but be aware that editing this file directly in the way you propose hasn't been tested. Once the change has been made you will need to reload the model into the data dictionary service. Probably the most predictable way of ensure this happens will be to restart Alfresco.
Hope this helps,
Roy