You won't do it during bulk import. Instead, use the "Import" feature during the Claim workflow. in the Advanced Configuration step of that workflow, there will be Import/Export links in the upper-right once you click on one of the templates on the left column. First, Export the sample file and then fill it in with all the devices and variables that you want, following the format of the sample. You can add devices that you plan to Claim later and keep using the same file. Once you import it, it will look through and populate the variables for that template for all the devices you are claiming.
Note that you can pre-Claim a device that is in PnP Inventory, even if it hasn't yet contacted Cisco DNA-C. In this scenario, once device boots and phones home to Cisco DNA-C the onboarding will automatically continue since you've already told it what config to use.
