Hi Adimatis
You misunderstand what an open keyboard is designed for, so here is a brief description.
1. If you have hardware arranger and you want to add sounds they have to be for that specific board. (Roland require Roland voices, Yamaha require Yamaha voices etc)
2. If you have a computer you can use all different types of formats of sounds, you just buy the appropriate software to play them.
3. If you have an open keyboard then it has its own sounds, just like any hardware board, but it also allows you to use other types of sounds if you so wish. (However other types of sounds should not be used as a substitute for poor internal sounds, but purely to enhance what is already available)
4. As open keyboards are software based you do not need to change them, but can simply upgrade the software to give you all the latest sounds and features. (Due to the advancement in technology you do sometimes also have upgrade the computer hardware, but as it is a standard PC this is quite cheap to do)
5. This is and always has been the Wersi philosophy since they first introduced the open keyboard over 7 years ago. (Wersi do not supply any other sounds then their own, if you wish to use others then you will need to purchase them separately)
Hope this clarifies matters

Bill