Sure, there are ways to do this quickly.
How many subnets are you needing to add?
Depending on your answer, we'll go with a) adding subnets by hand, b) subnet allocation wizard, c) IP Address import (and a couple steps therein).
How do I enable a Network scan? to retrieve the data, I think it would be best to scan the network and see what comes back.
How to add a singular subnet:
1) Ping a device you know is in use, responds to icmp, and is within the subnet you're going to add (do this from the Orion host to verify connectivity).
2) Add in all the likely snmp community strings the devices will have (within IPAM Settings page).
3) Click Add Subnet (within Manage Subnets & IP Addresses tab)
4) Enter the Subnet Address & CIDR
5) Click Save.
6) Select this subnet & click Scan. (This Normally not necessary, but click Yes when asked if you'd like to see the subnet scan status).
7) If any scans are taking way too long, click 'cancel' until your subnet starts scanning.
8) When the scan completes, you'll have the data you want within the Web UI.