siriofernando,
As I (mis)understand it (that you wish to obtain the same as in the first two samples), you can:
1) Select all the artwork and Group it, then apply Object>Path>Offset with an offset that is sufficient to create a continuous outer boundary (everything overlapping) of a suitably smooth shape;
2) Hide the original artwork in the Layers panel (the topmost Group), then select the offset artwork and apply Pathfinder>Unite/Add to shape area, then apply Object>Compound Path>Release, then apply Pathfinder>Unite/Add to shape area, to have one full path, then give it a white fill and a the right stroke to serve as cutline;
3) Apply Object>Path>Offset with a negative offset that is less than the offset in 1) by an amount equalling the desired width of the white backgraound.
Edit: And the necessary initial outlining that Monika mentioned while I was delayed in posting, the general approaches being similar.