passing multiple secondary geometries into vertex shaders using threejs

Lets say I have a geometry which I am using the vertices of to create Points or an InstancedMesh. But then I want change this underlying geometry to something else, let's as a cone to a sphere or something which has the same number of vertices. I would like to animated between these without using MorphTargets so I guess I need to use a custom vertex shader which is fine however I'm a bit stuck as to how to pass in the additional BufferGeometrys into the vertex shader. I can't really think how I might do this with the uniforms - has anyone got any ideas as, in my understanding i can only use int/float/bool/vec/ivec/mat but i need multiple vertex buffers - is it just an array of some kind?

I guess i'm trying to find a way of having multiple "full" geometries which i can interrogate within the vertex shader but can't figure out how to access/pass these additional buffers into webgl from three.js

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