@amandarosiak, there are a few ways you could do this with Celigo.
1. Ideally, you can get the fulfillment information from an api. Typically, a 3pl (I assume this is your use case), has apis you could connect to to get the data you need. That way you don't have to manually upload a csv file.
2. If the 3pl doesn't have an api, sometimes they can drop the csv file to an ftp server. From there, Celigo could grab the csv file and create the item fulfillment.
3. Lastly, if no automation is an option, you can use Celigo's bulk load. Once created, you would just go into your Celigo Flow and upload the csv file needing to be processed.
https://docs.celigo.com/hc/en-us/articles/226949488-Create-a-Data-Loader-flow
Lastly, like
@al3xicon stated, you would need to transform the sales order record to a item fulfillment. So in the mapping, you would utilize this help center article:
https://docs.celigo.com/hc/en-us/community/posts/360077054332-FAQ-How-can-I-access-the-Created-from-InternalId-in-NetSuite-on-my-import-mapping-.
PS - I've attached a data loader template here. Hopefully it helps (to install, go to the home page, top right click install integration).