From Marketo's perspective, SFDC account fields are available for use in triggers/filters on each Marketo lead (SFDC Contact) that belongs to that SFDC Account. So simply by having it at the SFDC Account level, it will be visible in Marketo and you can use it.
If you want it available on the Contact record in Salesforce for some reason, you could create a formula field on the Contact record and reference the Account field in it.
But just make sure to use the Account version of the field in your Marketo filters...formula fields don't trigger the Marketo/SFDC synch so you may not have current data for the formula field in Marketo.