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 Dynamics 365 data.
Assuming you have added a lookup to your data source back to CRM (see the guide CRM Lookups) to get the Entity ID Guid 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 entity where the name of the account equals the value.
LOOKUPB("accountid", "account", WHEN("name", value))
You can also lookup values in Option Sets by using the format
entity|optionset as the data source.
For example looking up the
id value for the "Consulting" industrycode in the account|industrycode optionset.
LOOKUPB("id", "account|industrycode", WHEN("name", "Consulting"))