Assuming you have the source information in your SCCM database you can modify your SQL Query and your Field Mappings accordingly, as mentioned by Pranay. A few additional tips:
1) Make sure you are using the Service Graph Connector for SCCM, not the legacy SCCM import, which has been deprecated and is no longer supported. If you are still using legacy SCCM import, then you should upgrade ASAP.
2) Make sure you have IntegrationHub ETL application installed, so that you can edit the field mappings in the SG for SCCM.
3) Make sure you make all of your changes in the SG for SCCM application scope as well. IntegrationHub ETL will allow you to make some modifications without being in the correct scope, and this will mess up your update set so that you won't be able to migrate it to your Production environment.
4) As for the classes to use, IMEI is supported on the base hardware asset record, but on the CI side it is only supported out of box in two specific classes, Communication Hardware and Handheld Device. So you will also need to modify your class mapping logic in the ETL field mappings so that these classes are used where applicable. Otherwise you would need to define your own custom fields to store the data.
