There is a built in Import Organizations tool that asks for Data to be in the following structure:

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Sample File

Sample Org Import.csv

NOTES:

Importing Custom Fields:

Add your Custom Field Set

When importing the Add Custom Field needs to be an exact match to your set. Best to take a screen shot of your Custom Field Set:

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On Import create the import items to allow them to come in. Use Exact Match on your custom field sets.

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