How could licences be chosen to allow this?
As an example, how are things done with paid addons with Prestashop?
According to the GNU Licenses FAQ here and I quote
However, in many cases you can distribute the GPL-covered software alongside your proprietary system. To do this validly, you must make sure that the free and non-free programs communicate at arms length, that they are not combined in a way that would make them effectively a single program.
So I think you can combine it with proprietary as long as it's in an arms length.
For eg, Plugins/Add-ons, but make sure the core working of your program is not completely depended on that plugin.
Some Open source licenses such as MIT or BSD does allow you to sub-licensing.
Again it depends on what license is used.
Disclaimer: I'm just a programmer, not a laywer.