Devices imported in with cm_data_import will be in Cm_computer_system but will not have a corresponding cm_device entry and as such no dev_id.
The reason alarms show in alarm console but not in USM is because they are missing a valid dev_id to map them to a specific device.
If the alarm is coming from somthing such as net_conning or icmp using the discovery agent from the hub where the probe is running will usually add enough information into the local niscache and database to allow the alarm correlation to happen.
If the alarms are coming in say from snmptd that and no other monitoring of the device is being done by UIM than you will probably not be able to make this work.
Alarm2 alarms come from either replication or from alarm_enrichment.
The only way you have access to the dev_id to update it is using alarm_enrichment, as alarm_enrichment can update any field in the alarm. Pre-processing and AO only have a subset of feilds they can modify.
But if a valid dev_id does not exist is the system with a matching cm_computer_system entry then you will not be able to get a proper mapping.
Hope this helps exlplain.,