Not a simple one because in each presentation internally the unique slide ID is used to direct the hyperlink. HOWEVER these are only unique within a single presentation so thet when you merge there would be duplicates. PowerPoint cannot accept this and creates new IDs which sadly will mess up internal links.
It is possible to write code to tag slides with the correct destination before you merge and then remake the links after the merge. Not simple though.
Shyam has an AddIn
http://skp.mvps.org/ihf.htm#.Vwkf-vkrK70 that tries to sort out the links and somewhere we do too (but right now I cannot find it!)
EDIT I found our AddIn drop me an email if you want to try it
(contact page on
www.pptalchemy.co.uk)