How can I add a custom command to Visual Studio?

All the configured external commands in VS are available to add to a Toolbar or Menu through the Tools>Customize menu as "External Command ##". You just need to know which number corresponds to the particular command you've configured. I believe it is in order of entry in the external commands list.

Similarly, you can find those names in the commands list in Options>Environment>Keyboard and assign a shortcut to your command that way.

First, add an item to your Tools menu by choosing Tools, External Tools, and filling out the dialog, like this:

Then bring up Tools Customize, click the Keyboard button at the bottom, and find the appropriate external tool number: