Tupi’s file is readable like synfig’s sif. It’s possible to create an importer for synfig via python for example.
The drawback of this is that you must be careful that you will be in the same scale (matrix) and that you will not have the synfig scene in tupi and the opposite when you will drawing.
As for just merging the source it’s not possible. It is not that simple like copy paste. The only benefit that you may get from tupi’s source code is ideas of implementation but you will have to extremely manipulate it, to merge inside synfigs code.
In the end you better write a new code for synfig to get a frame by frame tool.
I am not sure but I think that the problem of frame by frame animation it’s not the code but how to implement it inside the gui.
Also if I am not wrong a frame by frame (only bitmap) is in the writing process.
Check out this. http://www.indiegogo.com/projects/set-your-priorities-for-synfig-development-in-october-2013