I generally just use the predefined RFC 1918 objects in $HOME_NET unless you really intend to use the specific subnets in granular way.
If you do need them all for use in policies, the API can be used for this purpose.
The API guide is here:
http://www.cisco.com/c/en/us/td/docs/security/firepower/621/api/REST/Firepower_REST_API_Quick_Start_Guide_621/objects_in_the_rest_api.html#id_19416
If you would like some background on using the API, there are some third part resources at the following:
https://www.youtube.com/watch?v=1fsgGnant1U
https://github.com/CiscoDevNet/fmc-rest-api/blob/master/labs/firepower-restapi-101/2.md
The API Guide describes the following:
PUT networks
Request Type: PUT
Description: Modify network objects.
URL: /api/fmc_config/v1/domain/{domain_UUID}/object/networks/{object_UUID}
Permissions: Object Manager>Modify Object Manager
If you would like some background on using the API, there are some third part resources at the following:
https://www.youtube.com/watch?v=1fsgGnant1U
https://github.com/CiscoDevNet/fmc-rest-api/blob/master/labs/firepower-restapi-101/2.md
