Ops Manager does not have a bulk loading option for custom comment fields.
But you can use the 'dfm comment set' cli and create a script to perform bulk load (i presume you want to set the comment field for user objects in OM):
STEP 1 - Export the list of users
$ dfm report -q -F csv users | awk -F "," '{ print $2 }' > users.csv
STEP 2 - Edit the csv file to add a second column which is the cost center. The file should look like:
root,1001
wheel,2001
STEP 3 - Set comment fields in OM (replacing <comment-name> with the name of the comment field you have created)
$ cat users.csv | awk -F "," '{ system("dfm comment set " $1 " <comment-name>=" $2) }'
STEP 4 - Verify the values are entered
$ dfm report users-comments