Kobo and Kindle

I've downloaded both Kobo and Kindle apps to my phone. I've had Kobo for awhile and I like it but I thought I'd try Kindle as well since it is also free.

I don't know if I'll buy any books but I will take a few of the free ones.

What I am wondering is. Is it possible to transfer books I have downloaded on my computer to either the Kobo and Kindle app?

I do have books on my phone like I mentioned the other day and I can read them just fine and dandy.

But. If there is away to transfer them to either app?

Well. That'd just be nifty.