When you create a network dataset, you specify the line/road features, attribute and restrictions, connectivity policy, etc. These settings are the schema of the network dataset. You can export the schema, and use it to create another one. For example, some one sends you an updated line/road feature class. So instead of going through the property pages and re-defining all the attributes/restrictions, you can use the schema to create a new network dataset on this updated road feature class. Everything about the input data and the fields that take part in the schema must be the same.
If the data for the island and then the data for the rest of the country is in the same format then you can use the schema from the island to make a network dataset on the country. All the schema does is save you steps in re-creating a new network dataset.
Do you already have the road data from all the country? It might be easier to use the Ready To Use services from ArcMap and do your drive time analysis on the street data we host via ArcGIS Online. In the Catalog view, you can expand the Ready To Use item and find the logistic services for Service Area. You will need an AGOL account and will have to use some credits to use these services. Info here: