I understand that you do not have set up the standard Marketo - SFDC sync and that you are using a export/import process to sync data between the 2 products. Am I correct ?
On step 2, ou say that you set the lead source info during the export process. Are you setting up the value of the lead source field in Marketo ?
ON Marketo, you can lock field updates in the admin -> lead management section, so that once the field has a value, it cannot be changed in the furture.
You could also use your smart campaigns that you use for the Sync to say 'Lead Source IS EMPTY' and that would only populate that field if there is nothing there.
And then you could also use the choice if 'Lead Source IS xxxxx' then update the new field you've asked for (we use Lead Source - Most Recent', and this populates each time.