You can create a VM template from a VM’s disk. While the process is somewhat tricky the goal is achievable.

Acropolis Image create, update, delete (CUD) behavior has changed in AOS / Prism Central 5.5. Image API CUD behavior depends on whether a cluster (also known as Prism Element) is registered to Prism Central.

• If the Prism Element (PE) cluster is newly created and has never been registered with a Prism Central instance (that is, never managed by Prism Central), all image CUD operations will be allowed on the PE cluster.
• If the PE cluster is registered with a Prism Central instance (that is, managed by Prism Central), CUD operations will be blocked on the PE cluster. Ownership of images is migrated from PE to Prism Central. CUD operations must be performed through Prism Central.

When CUD operations are blocked on Prism Element, they cannot be performed through the version 0.8 API, version 2.0 API, Prism Element web console, or acli. These CUD operations are replaced by the Prism Central version v3 API, Prism Central nuclei command line, or Prism Central web console. However, the ability to create an image directly from a specific VM's disk is not currently available.

For a how-to please read KB-5033 Prism Central: Create an image from the disk of a specific VM

Spoiler alert: the KB is guiding on obtaining a URL of the disk to be used in PC when creating a VM image.