I've search for this and only been able to find the reverse (i.e. add a custom function to an existing ribbon tab/group).
We have a custom ribbon that we want to add some built-in functions to, e.g. we have a few custom number format buttons and beside those, I want to add the regular built-in increase/decrease decimal places buttons so that user doesn't have to return to the Home tab to change decimal places.
How do I add these built-in buttons to my custom tab/group?
I'm on VS 2017, and so far, my method has been using the visual designer to add controls and then clicking through the 'view code' option to code the functionality.