The Constituent ID exported in NetCommunity corresponds to the System Record ID in The Raiser's Edge.
The Raiser's Edge only uses the Import ID, Constituent ID, or Social Security Number to identify existing records.

1.  Create a new Constituent Query
2.  Select Constituent Information > System Record ID from the list of available fields
3.  Choose 'one of' as the operator
4.  Enter each 'Constituent ID' from the NetCommunity export
5.  Output either the Import ID, Constituent ID, or Social Security Number
6.  Run the Query
7.  Export the Query as a .csv file by selecting File > Export from the menu bar at the top
8.  Use exported spread sheet for Import