Yes, and the UK for example has already done so. But that kind of arrangement would create a lot of red tape. Sweden and Finland are de facto under NATO protection already, and the main benefit of formal membership comes from NATO's coordination structures (planning, logistics, operations etc.). With 28 separate treaties, things would be far more messy and complex.

Of course, if Turkey doesn't see sense, Sweden (and Finland) will be integrated into the collective defence system. NATO is the preferred option, but alternate solutions exist.