Thank you for asking. There is not possible to merge trees on MyHeritage. You can merge 2 GEDOM in Family Tree Builder. Details can be found here: http://www.myheritage.com/help/2008/10/is-it-possible-to-merge-two-gedcom-files-together/en/
Smart Matches do allow you to learn new information about your family from other tree owners. When you get a Smart Match, you can merge new information about the relevant individual to your tree.
But this merging operation is done over facts of the matched individual only, and you must go through the matches one by one, that is you cannot automatically merge 50 individuals to your tree from another tree.
There are a couple reasons why we won't allow this:
* Such a feature can damage the privacy of other tree owners. We only let you see information about other individuals if they also appear in your tree.
* Some genealogists do not like it when someone can take they're whole research with one click. Smart Matching allows you to build a relationship with other site managers slowly. You can always request to become a member of their site and gain access to their whole tree.
I hope this clears things up, let me know if you have more questions.