You can use Data Sync to lookup values from various sources, for example a CSV file, and insert the specified fields/data into your source Salesforce data.
Assuming you have added a lookup to your data source back to Salesforce to get the Object ID value for the related item you can build the value like this:
Or even via in-line LOOKUPA/B functions using calculated columns:
Using LOOKUPA/B Functions to get the accountid from the account object where the name of the account equals the value.
LOOKUPB("accountid", "account", WHEN("name", value))
The video below demo's how to import account and contact data into Salesforce and how to apply a lookup between account and contact using Data Sync.