re: why is that even a mapping option.
One can map to the Entry ID column to use a CSV file to update preexisting entries. The entry ID column would have to be populated to the actual entry ID in Connections as shown on the Connections : Manage admin page. This feature was developed and released because it was one of the highest requested features for the CSV Imported.
re: So I have to create a custom field for the chambers ID number field?
Yes, or, use another field that you will not be utilizing and use the Say What plugin to change the field label.
Hope that clears that up better.