If you're not using any of the integrated POD services (Printful, Printeers and CustomCat), you can still use your preferred Print on Demand service with Zakeke.
By manually uploading the customizable products and creating a workflow with Order Desk it is possible to automatically send the print-ready files to the POD company.
These are the steps you need to follow:
First, upload the products as standard products on your store.
When you set the print area, make sure you follow the size guidelines that are provided by the print-on-demand company from which you're taking the products. Each company usually have his own guidelines.
Configure Order Desk in order to have the print-ready files generated by Zakeke automatically sent to the POD for fulfillment and shipping.
Order Desk works with most popular PODs, see the complete list here. However, even if the service you use is not supported, you can still make a custom integration via API (get in touch with Order Desk support for this).