You are able to update products Swift Distribution Flat with Stock Sync. Just choose on Swift Distribution Flat template after choosing to update existing products in your store. 

1. Go to Setup New Feed > Choose Update existing products.

2. Choose or search for Swift Distribution Flat template.

3. You need to fill up the Username and Password from your Swift Distribution Flat profile. 

4. Click Next to the field mapping section.

These are the available fields that can be updated with Stock Sync. 

SKUVariant SKU
quantityVariant Inventory Qty