For recurring donations, Donorbox has a true / false attribute called Is Recurring Donation. You can map this attribute to a Salesforce field of your choosing. The mapping option is under the standard field mapping page: https://donorbox.org/org/sf_setting.
Note that we do not utilize the SF NPSP's Recurring Donation object. The NPSP Recurring Donation Object is too convoluted to sync. Further details regarding data mapping can be found in the following guide: https://donorbox.org/nonprofit-blog/salesforce-data-mapping-guide/