Well, I know of no existing software program that will do this, but this is an easy program to write, in any language, if you're a programmer.
Basically, you just open the original PGN file for input, read and print to a new file the PGN tags, read and print the first six moves, and then go on to the next file. Continue until end of file. Done.
If you're not a programmer, just about everybody knows someone who they go to when they have a computer problem. Your own "computer guru" will almost surely be able to handle this simple programming task. Ask him (or her) to write it for you.
(I'd do it, but I'm currently in the middle of writing three other programs.)
If your PGN file isn't too large, I suppose you could always do it by hand, within TextPad, or your favorite text editor.