I'm migrating from one Netsuite account to another using variuos csv imports. Question is there an easy way to link sales orders to invoices? I already know that its not possible via csv import. Also I guess WF will not help too, because I need to link but not to convert SO to Invoice. Do I need to involve scripting?

You can't do this after the fact

how about a custom field for the source transaction? You can at least upload that data into your invoices and SOs

Yes there is a way. I just did it for a client Create sales order Then use a script to create invoices I had my dev write a script to take the invoice file from A folder in file cabinet and create invoices like someone would create from UI It had billing schedules and all too. Just needed to make sure invoice lines were mapped to S.O lines perfectly. This will have a huge impact on both revenue and total billed against a S.O so ensure to link the 2 transactions.

As Sham said, the best way would be via a SuiteScript on creation of the Sales Order. If field is checked "Create Invoice" then it creates the Invoice (and possibly the Item Fulfilment if you use stock). You could also have custom fields on the SO if there are unique values (e.g. if the Invoice/IF date is different from the SO date, etc.). If you accept payments against Invoices, you could in theory create a SuiteScript on the Invoice record on create to generate these too. Yes, it involves some extra work to begin with but it could be pretty slick and you only need to import a CSV with the SO/Inv/IF/Payment data.