I am trying to do something similar- import from another asset management service where I have a handful of custom fields. I can create the same custom fields in Service Desk, but I cannot migrate/map the old data fields to the new system custom fields. It's hard to imagine why using a CSV to import and map would not allow one to use new fields. Might be a deal breaker if we can't bring over all of our asset data, and I'm not going to manually enter 2500 records' custom fields.
I did something similar using PowerShell and the Samanage API. I'm basically reading from a CSV export of one system, iterating through the list, and using the API call to map the desired fields and create new assets.
I'm happy to share a code sample and walk through how to get started using the Samanage API if you'd like. Just let me know!
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