Unfortunately there are some commands that cannot be found in the Hotkey Editor. I'm not sure why, but I do know of a quick fix.
- Open the hotkey editor.
- Click on the Runtime Command Editor tab above the keyboard diagram.
- Click on the "New" button. This lets you create a custom script to set to a hotkey.
- Give the custom script a name, like SnapTogether. The name can only have letters or underscores.
- Copy this line of code and paste it in the large text box where it says to enter your command. This is the code that Maya runs when you select the Snap Together Tool from the menu.
setToolTo snapTogetherToolCtx;
- At the bottom-right corner of the Hotkey Editor, click Save Runtime Command.
- Now, in the "Edit Hotkeys For:" dropdown menu, choose Custom Scripts. You should find the new custom script you made, to which you can assign whatever hotkey you want. You can also search the name of your custom script and find it that way.