for non-free extensions you'll have to make a script that downloads the file and installs it, unless the foxit people give you permission to redistribute it. this is done with skype and flash.
Personally, I wasn't thinking of making an extension for it anyway. I just thought some others might find the program useful. I do note, however, from comments on the Puppy linux forum that they have been in touch with the developers, whom, I infer, seem to be quite happy at Puppy providing a dotpet of the product to their users (indeed Foxit software mention on their official website that their pdf reader is being used successfully by Puppy).
. . . the program seems to be way better than epdfview.
an opinion you're certainly entitled to. personally speaking a program is automatically inferior by my standards if i can't legally make copies of it for friends, or even mirror it online. and that's not even taking into account the i-can-look-at-the-source and i'm-allowed-to-modify-it issues.
Many would agree with you, i'm sure, though I'm not much of a purist when it comes to such matters. Of course, it is nigh on impossible to ultimately trust any software which isn't open source; such software is only useful for as long as it is supported and works (I make no ethical judgement otherwise, except to comment that I believe that we all deserve to be able to make a living somehow - even programmers... I doubt many purely open source programmers manage that), but for now I am happy to use Foxit Reader in preference to the other possibilities.