I would check the records with blank 'Sales Owners' in SFDC. Most likely they belong to lead queues not SFDC users so aren't 'owned' by single person for personalization. For example you likely wouldn't want an email to come 'from' "Qualified Lead Queue - US".
Marketo cannot write to these two fields that you indicated they are Read only fields. Another way to go about this is to create a custom field "Uploaded Sales Owner". Then you can create smart lists and part out the data however you wish with a single upload.
Then you can import them in and have that data. You stated that you don't have an integration with SFDC but you do have SFDC. If possible, I would highly recommend integrating the systems.
I had the same question but we integrate with Microsoft Dynamics CRM so this is helpful. Thank you.
Does this mean that it is not possible to update/upload the contact details of users?
What I'm trying/investigating into is the ability to update and pull the direct phone number and mobile mobile number of the sales people from Microsoft Dynamics. It really looks like the options in MD are so limited on the user level.
Any help would be great. Thanks.