You can try something like this
LPDIRECT3DDEVICE9 l_Device= RenderManager()->GetDirectXDevice();
LPDIRECT3DSURFACE9 l_RenderTarget, l_Surface;
m_Texture->GetSurfaceLevel(0,&l_Surface);
l_Device->GetRenderTarget(IdStage,&l_RenderTarget);
l_Device->StretchRect(l_RenderTarget,NULL, l_Surface,NULL,D3DTEXF_NONE);
l_RenderTarget->Release();
Where IdStage is current render target you want to copy, in your case it will be 0
m_Texture is a DirectX Texture that you want to receive the copy from the backbuffer