I'd like to add a lock tool to my building game but the ones from the catalog don't work anymore since ROBLOX disabled the use of tools without handles. Is there any way I can add a handle to the HopperBin?
Not directly, no, because that is the chief difference between HopperBins and Tools.
If you want to use a handle, you need to use a Tool (which is a really simple conversion if necessary).
Of course, you can always modify the posing of the character and weld an object to their hand, but you will have a lot of work to make it be as fluid and provide the full functionality (e.g., dropping and picking up).