This merge is implicitely supported by the 'if' expression.

   <if>
      <equals field-value="category" other-value="new plant" />
      <participant ref="reviewer-b" />
      <participant ref="reviewer-c" />
   </if>

In this example, the 'merge' happens at the end of the 'if' expression : either the branch 'reviewer-b', either the 'reviewer-c' comes back and makes the flow continue.