I gather that you would like to use the foreign key do deduplicate ?
This can be done if you update leads or import leads through the API, but not though Marketo UI.
Vote here : Enable to use other matching UID field on import
That is what I thought. I was in fact trying to do this using the API but with importToList not syncLeads (I know this can be done with syncLeads). I need to use importToList because I cannot set the aquisition program with syncLeads and the problem I have is that importToList does not return the marketo ids as syncLeads does. To complicate matters we are required to load leads without email addresses.
In any case I have found a solution but thanks anyway.