That is a key. You may do it by modify sources of nopCommerce (not recommended) or by developing new plugin (recommended). To develop new plugin I would suggest to move first steps to doc, as well as you may look at default nopCommerce plugins to check/copy & modify existing one.
I have been trying for sometime to modify existing code or create plugin from scratch and extend product functionality but the documentation doesn't really help.
Did you look Specification Attributes - are they any help ?
To add a new field: Unless you are planning to learn how to make a plugin then you need to change the core code Search the solution for "OldPrice" which is unique. Study everywhere OldPrice is used and deturmine if you need to modify that piece of code for your new field.
Then you need to add a new field to the Products Table called "AnotherPrice"and then Then every where OldPrice is used add the code for "AnotherPrice"
There are other ways to do it depending on what you want to do with the field I.e. rather than modify the Product Table you could use a Generic Attribute