The SFDC Id token can be used in emails and the value can be viewed in a Smart List and on individual records. You unfortunately can't search by a SFDC Id in the database area of Marketo.

 

The SFD id can’t be used in a Change Data Value flow step to populate a custom field. The SFDC Id also can’t be used as filter in a smart list. You can populate a custom string field with the 18 character SFDC id using the tokens {{lead.sfdc lead id}} and {{lead.sfdc contact id}}.

 

Flow example

If you have a large database, I recommend running 100K batches at night and on the weekends and waiting a few hours after each is completed so you don't impact system performance.

 

By the way, after a lead is converted to a contact you will need to run a daily batch campaign to set the new SFDC Id.

 

Here is an example of how to set the Smart List of the batch campaign.