Change the category for this product group to a category that does allow variation listings on eBay. Your eBay category is in the Category & Description tab when editing a SKU. This is a group attribute, so changing it on the User Interface once changes it for all SKUs in that product group. If you are going to edit this using the import / export feature, make sure to edit the eBay category for the Master SKU. Here's a useful page of multi-variation eBay categories.

Ungroup your SKUs on Zentail so that they are single, stand alone SKUs. This will potentially break up the product group on any other channels or marketplaces this group is listed to through Zentail.

Create an alias of each of the SKUs within the group. Maintain the alias SKUs as single, standalone SKUs and set them to list to eBay. To do this, you may want to clone each SKU first, and then make the clone an alias of the respective SKU from the original product group.