Is there a way when doing a mass import of all our devices to populate the asset LifeCycle -> Event and End User Fields. I don't see an option in the d42_generic_import_tool_v7.3.0

You can accomplish this by downloading:
Create or update lifecycle data >    Download Sample Excel File 

And inserting the devices name under "device" column or IDs under "device_id" column, you need one of them, then insert the lifecycle event data and import.
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and here the result:
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For "Assets" we need to use the asset_id field and insert the ID of an asset.

Note: The end user must be exist on Device42.

You can refer to this page for all detailed fields required:
https://api.device42.com/#create-lifecycle-events