Nextruder on MKS3.5s
It should be possible to simply assemble the Nextruder and use with MKS3.5s, the connections on the Buddy board is there already.
RE: Nextruder on MKS3.5s
From my experience running 3 Mk4S and a Mk3.5, I can't help but wonder why you would consider downgrading your Mk3.5S....
My Mk3.5 is set and go. My Mk4S's need constant baby sitting with their "never perfect first layer". I'm one of those unfortunate ones affected by the nozzle issue that has been discussed for five months on GitHub. But obviously there are many other users whose printers don't show that issue, so take my rant with a grain of salt. But I think you should have an excellent reason why you would risk modifying a working printer unless you're really into tinkering.
Formerly known on this forum as @fuchsr -- https://foxrun3d.com/
RE: Nextruder on MKS3.5s
That is 3.9 upgrade. I decided to upgrade my mk3s to mk3.5 only because I had REVO, so without the Nextruder that was available in the 3.9 upgrade.
This because I also purchased an MK4S, so now I have two printers.
It should be possible to simply assemble the Nextruder and use with MKS3.5s, the connections on the Buddy board is there already.
RE: Nextruder on MKS3.5s
Yes, the xBuddy board in the MK3.5S has the necessary connections for the Nextruder, making the hardware integration feasible. However, Prusa doesn't currently offer an official upgrade kit from MK3.5S to MK3.9S or MK4S. Upgrading would require sourcing parts like the Nextruder, LoveBoard, and cables individually. Some users have expressed interest in such an upgrade path, but it involves significant effort and cost. Unless you're keen on tinkering, it might be best to wait for an official kit or consider other upgrade options.