Personally I do just edit the merge buffer directly, but you can teach Emacs what to do if you have a standard requirement.

<kbd>+</kbd> calls `ediff-combine-diffs`:

> Combine Nth diff regions of buffers A and B and place the combination in C.
> N is a prefix argument.  If nil, combine the current difference regions.
> Combining is done according to the specifications in variable
`ediff-combination-pattern`.

which is:

> Pattern to be used for combining difference regions in buffers A and B.
> The value must be a list of the form
> `(STRING1 bufspec1  STRING2 bufspec2 STRING3 bufspec3 STRING4)`
> where bufspec is the symbol A, B, or Ancestor. For instance, if the value is
> `'(STRING1 A  STRING2 Ancestor STRING3 B STRING4)` then the
> combined text will look like this:
> 
> STRING1  
> diff region from variant A  
> STRING2  
> diff region from the ancestor  
> STRING3  
> diff region from variant B  
> STRING4