As a Roblox developer, it is currently not possible to insert Motor6D instances via the Advanced Objects panel in Studio. It is simply not there.
There are cases when developing where I need to insert a Motor6D instance and cannot without using the command bar.
One such case is building subrigs on my custom characters. I typically rig my character separate from the Torso, Head, etc cluster of parts. This cluster of parts doesn’t require detailed rigging since I make them invisible, so a default state Motor6D is fine and would be quick to insert if the Advanced Objects panel contained it.
The Advanced Objects panel should be able to insert any and all instances available to me in Studio, including Motor6D. I am unaware if there are other missing classes, but all creatable instances should be insertable via this panel.