Accessing Metal Backend MTLCommandBuffer, MTLRenderCommandEncoder, MTLDevice from the engine ( iOS )

[orassemusse]

Hello,
I want to draw objects from external library like UI and particule effects such ImGui, Effekseer, etc that require acess to a metal MTLCommandBuffer, MTLRenderCommandEncoder, MTLDevice on iOS. For exemple I use:
ImGui_ImplMetal_NewFrame(MTL::RenderPassDescriptor* renderPassDescriptor) and ImGui_ImplMetal_RenderDrawData(ImDrawData* draw_data,
MTL::CommandBuffer* commandBuffer,
MTL::RenderCommandEncoder* commandEncoder) to draw ImGui from a simple renderer.
How can I access the metal backend from the Filament engine code ? Do I need to think about render pass, where and when to draw ?
best regards,
Olive