You add files to storage memory not to the rom. The only way you change the rom is with a flash and you can flash a custom rom made by a chef or you can build one yourself using a kitchen.
ActiveSync lets you sync files from your device to your pc so when you add a mp3, document, pictures, etc on your device or on your laptop the sync updates both your device and pc with the same data.
You can use cab files that are downloaded directly to your ppc to install applications or you can download an exe file that will install an app on your ppc through the usb that is connected between your pc and your ppc.
You need to have ActiveSync installed on your pc to sync if you have WindowsXP installed. If you have Vista or Windows7 on your pc then you need Windows Mobile Device Center to sync with your ppc.
When you install those programs and you sync for the first time you can select what items you want to sync between your pc and ppc.
