I thought this was possible with a message but I can’t seem to find anything now. Perhaps I’m getting confused between "send backwards" and "add to background". However it does seem that if you add an object to the background and then remove it from the background it ends up behind other foreground objects, so a workaround might be something like the patch below.
– Pasted Max Patch, click to expand. –
Copy all of the following text. Then, in Max, select New From Clipboard.