Assuming your .prc books are DRM-free (I don't remember the DRM situation with .prc). You have a couple of choices:
1) Use calibre and convert them to a format that Stanza can read then load the Stanza app.
2) What you want to do is called sideloading. Do a search on that and you will find lots of info. One post is here: https://www.mobileread.com/forums/showthread.php?t=89040
it talks about a technique that works only if your computer is a Mac. I have tried it with a PC, and it didn't work.
3) You will also find another sideloading discussion that seems to involve jailbreaking your Ipad.
4) I haven't tried this, but I wonder about emailing the file to your Kindle? What would happen if you used that and tried then to open it in iPad for the Kindle. I've never used this so I have no idea if it would really work, and don't know about trying it for kindle on iPad. Even if it does work, it seems a pain if your have a lot of files.
p.s. I should note that most of these ideas are not mine. I've just listed some of the things others have noticed about sideloading.