Can I connect a standalone repeater to a Nebula managed AP via WDS?

No, because Nebula APs use Smart Mesh to build up the connection within the same site.
The standalone AP is not able to connect to a Nebula AP via WDS.

Currently, Smart Mesh is a free feature and supported by all Nebula capable APs, so it's recommended to add the APs to Nebula on the same site.

To learn more, please visit: Setting up Smart-Mesh on Nebula-Accesspoints (Root/Repeater Accesspoint)