This is correct.

The basic (or compulsory) part of the insurance is also the "social" side of it.
The supplementary is the private sector.

This is why every company has to give strictly identical cover for basic insurance whereas in the supplementary they can cover what they like (though they're all going to try and remain competitive obviously).

The basic insurance covers any pre-existing conditions that you may have so no worries there but Swisspea is right. The chances are that you'll get refused for supplementary insurance.
You can always try and get cover. They may accept you for it but put a "reserve" on the diabetes. This means that it would still be covered under the basic but not under the supplementary.
If you were to need the supplementary for anything else (not related to the diabetes) it would still be covered.

This is a big "maybe". They may or may not be ok to give you the supplementary with a reserve.

One thing you should think about when applying for the basic part is your deductible. If your average annual costs are (or are likely to be) high then you may be better off with a 300.- deductible.

For more info. on deductibles :

Deductible / Excess / Franchise - which is the best one to choose?