The simplest way to do what you are suggesting is to add the wheels to the current frame, you do not need to build a new fork. All you need to do is arrange brakes. But the bottom bracket height is an issue. Best bet there would be either to make an adjustment to BB height, or to rear drop height. The latter will change quite a few things. So an option would be to consider the geometry of the current bike and see if there is any position you can scab another bb to. Probably hole sawing through the seat tube and inserting a BB into the seat tube, then adding a triangular brace to the DT would do it.

I have not even considered the chain line...