Even if you are using a webflow navbar object you should still just be able to drag a button into the element and place it/style it as you need with the normal css options - allowing you to round corners etc. If you already have a button style you can just apply that to the navbar one you create so they all look the same. Then just change the button text.
The pre-made elements that you can use from the webflow interface just have initial common formats that are helpful as a starting point, but virtually everything can be deleted or customised as you like - including adding any kind of element you need.
Hope that is what you wanted to know.