Hi Malik,
Fields created in Marketo will only ever exist in Marketo. If you need a field to sync between Salesforce and Marketo, then it's best to create the field in Salesforce first. It will then automatically map to Marketo as long as your sync user in SFDC can see it.
In this case, because the field already exists in Marketo, you'll need to create the field in SFDC and let it sync over. You'll then have duplicate fields in Marketo, and you have two options. You can hide the Marketo only field, or you can open a support case to have them merged.
John