Hi,
You could write a C# application that can fetch your data using normal SQL calls and connections. These data sets would need to be mapped into CRM Entity objects and then created using the standard CRM web services.
You can download the CRM2013 SDK for more detail on how to construct these objects.
Plugins are not really used for this kind of thing (although its entirely possible), the objects you would use in the plugins are very similar to the ones in the normal C# application.
Hope this helps.
Don
PS just in case you don't know direct SQL update/inserts/delete into the CRM data are unsupported by microsoft.