Product with additional "sub products"

It sounds like you might be talking about grouped products? (Go to the link, then click on Adding a Grouped Product in the left-side menu.)

Actually no, ’cause the ‘parent product’ as the ‘child product’ must be sold… Imagine the product as a device that has some modules wich could be plugged on it. The main product would be the device, and the child products would be its modules, that could be bought or not with the main product, and both could be sold separately..

https://www.woothemes.com/products/composite-products/
https://www.woothemes.com/products/product-bundles/

Thank you! This composite products will be very useful!