Yes, you will either need to learn how to use the Document Manager API or the SolidWorks API to do this.
The DM API lets you access custom properties with having SolidWorks installed or opened. It also requires a knowledge of VB.NET and stand-alone creation in Microsoft Visual Studio, so while it can offer greater performance and flexibility, it is considerably more difficult for someone new to programming. (The DM API can be used while SolidWorks is open, but this can cause problems if you're not very careful, plus if you have SolidWorks already open, then why not just use the SolidWorks API which is much simpler?)
Using the SolidWorks API, the document will need to be opened and you will need to use the ICustomPropertyManager interface.