With VSTO there are two options:
- using the Ribbon Designer or
- implement Ribbon XML manually.
With the first option you get some nice tooling to add and create your Ribbon visually, with the second you will dive deep in and build your custom Ribbon XML code by hand.
The benefit of the second option is that you'll get complete control over everything that is possible with Ribbon XML and won't be limited to what the UI provides to you (RibbonXML can be very extensive, there are huge bibles covering all of that in detail, sky is the limit).
Here are two walkthrough documents that guides you to both options:
Walkthrough: Creating a Custom Tab by Using the Ribbon Designer
Walkthrough: Creating a Custom Tab by Using Ribbon XML
Your other question is, can it be positioned in existing Ribbon tabs ... Yes it can, if you select the fixed Ribbon XML tab IDs you can mix in with existing Tabs:
https://msdn.microsoft.com/en-us/library/bb608593(v=vs.120).aspx
Of course, all of these links are starter pointers, but just start trying to pick one of these and see where you hit a roadblock. That gives you the option to ask a new question with more detail and slowly grow into a full working application!