That rubber insert is heat molded into the clip on during manufacturing of the clip on.
If your rubber insert has become dislodged, you can get that clip on replaced under warranty. There is no good reason for that to break loose accept a manufacturing defect.
That rubber insert is needed to install the HP billet lever guards. Those lever guards add to the weight of the clip. Then there is a weight that can be added with the lever guard that adds additional weight to the clip on bar end. All this hangs on that rubber insert.
You mention expanding inserts. There is no expanding inserts with the BMW HP guards for even the K46 that came from BMW.
If you are asking if there are inserts that do expand into the bar end and allow the mounting of that lever guard, yes there are. I have purchased some from eBay several times when I was testing the WoodCraft mounting solutions and clip ons. I prefer the OEM clip ons and the mounting methods.
It is possible to at least try JB Weld on the rubber insert and put up inside the clip on if you really dont want to claim it under warranty. Or even buy a clip on for $75 off ebay is the better way I would do it.