Supplying a filename to a file open dialog

What I am trying to accomplish is to create a new PowerPoint
presentation file based on several existing PPT files.

Thanks to PreFab I have the dialog for the “Insert -
Slides from file…” dialog opening, but I don’t know how to get a
file name into that dialog and to ‘click’ the OK button. I’d have a
list of file names and will eventually use a REPEAT loop to do this
the appropriate number of times. But I’d love to just get one of
these scripted to get off the ground.

Here’s what I have so far:

tell application "Microsoft PowerPoint"
set newPresentation to make new presentation
activate application "Microsoft PowerPoint"
tell application "System Events"
tell process "PowerPoint"
click menu item "Slides from File..." of menu 1 of menu bar item "Insert" of menu bar 1
-- How do I get a file name (that would be an AppleScript variable) into the dialog here and click "Okay"?
end tell
end tell
save newPresentation in ¬
"Macintosh HD:Users:freejak:Desktop:ora.ppt" as save as presentation
quit saving no
end tell