I know exactly what you mean and it would be nice to have. However, this is not under my control. It’s a request for Scirra to allow other objects to be used as textures for the 3D Shape. Please contact them though C3 forum and their aha suggestion platform. I am also on the Construct Community Discord.
An alternative would be to create a C3 3D billboard plug-in which accepts other textures, but that’s a bit of work to add another 3D plug-in just to work around 3D Shape limitations.