It’s not possible yet in LibrePCB, but you can do it with external tools on the Gerber files. See for example in Gerber Panelizer: How to panelize boards.
Only downside is that with this, the LibrePCB Fab service cannot be used for ordering since you need to order with Gerber files.
Alternatively and more easy, use the panelize feature of the manufacturers. I think at least PCBWay offers this option during the order process, but I didn’t check.