Is there's an API to create Work Order ? I've been asked to create a script to import data from excel and create work orders

Yes there is, and you can use data loader from integrator.io if you want

You can also use CloudExtend Excel Data Management for NetSuite and skip creating a script. Free trial and free support

WO is just a transaction record, like Sales order

@reza.seedin its not available via CSV right now

I would create a Suitelet=> then process the CSV data as Script and create the WO