I don't think that it is possible to do that using formatting switches on
the field.  This means that you would either have to do modify the data
source, or after executing the merge to a new document, do a wildcard Find
and Replace where you search for

([0-9]{2})([0-9]{2})([0-9]{2})

and replace that with

20\1/\2/\3

To learn about use a wildcard search, see the article "Finding and replacing
characters using wildcards" at:

http://www.word.mvps.org/FAQs/General/UsingWildcards.htm