It is not possible to transfer your existing products from the basic PayPal widgets to the Online Store. The reason is that in order to do this, you will need a file of your products to upload (such as a .csv file) and the basic PayPal widgets don't support this.
If you do have a .csv file, you can upload it to the Online Store! This means that you don't have to upload each product separately. If you don't have a .csv available, you will need to upload your products to the Online Store. However, once you have done so, you will be able to perform a variety of functions not available with the basic PayPal widgets and will be able to take full advantage of the advanced product management features available to you.
To get started with adding products to your Online Store, take a look at the Getting Started tutorial.