I would like to add a column to a sharepoint list which is created in visual studio. am not using list definition, only list instance. I could open the list xml and add new items, but how can I add new column to the list? what's the best approach?
As always in such situations, the best option IMO is to avoid using declarative (XML) approaches to use instead a feture event receiver and C# code (unless you're developping an App).
Here's a good start: https://stackoverflow.com/questions/19548874/automatically-creating-a-list-and-a-folder-when-activating-a-feature-in-sharepoi/19566648#19566648. The code is to create a list, but it is the same concept as to add a field on an existig list.