Skyvia for Salesforce

Skyvia for Salesforce

Hello,
I have 2 tables I'm pulling data from to push to Salesforce using Skyvia Datasource method. I am having extreme difficulty getting data from the second table as the required fields in Salesforce do not have a correlation to the table I need to use, that info was already mapped from the other table. Is there a way to just update info in only the fields that have no value yet?