If you mean, can a pickup be used as a driver...no it can't.
Are you making the pickups yourself? You are probably aware that most of my drivers have been conversions of single coil pickups and my later ones virtually invisible so this may be possible and no real reason that it won't drive 7 strings, it is only lack of a market that they don't make them anymore commercially. I get a lot of enquiries about 7 string versions so yours would get a lot of interest.
If you are using three pickups...you will have to be clever with the switching...something that I always seem to have problem with. A super switch selector might be an interesting way to switch it, the sustainer being one of the selections.
Using the fernandes, it does apparently have a digital switching circuit but the details are secret so I can't really help with that. If you want to use this circuit and not make your own along the lines of the DIY version (which is what I would have suggested) you will need to work out a way to replicate the sustainer design.
The fernandes sustainer uses the driver with an active circuit so that it can be used as an active neck pickup and completely replaces it. It is unlikely my drivers would work well if at all in this way yet another reason to do your own circuit making your new purchase redundant.
So, post a few more details of the project, pickup types and and what you are wanting to do. We can but try to help...if anyone else has suggestions, feel free to chime in!
pete
The pickups & driver will be built by custom pickup winder Will Boggs @ WB custom pickups.
I was told single coils in bridge may not push the driver ( I see no reason why not) I do want the system stealth .This one from ebay is from sustainiac not Fernadez so I am not sure what i got.
Thanks Flick