There are three ways to answer your question.
1. No, there are no separate composition textbooks that focus on Koine; they are all focused on Attic, but that is okay, because Koine and Attic are not separate languages. These terms really refer only to differences in style. The comp books focus on vocab and forms and constructions that occur often throughout Greek literature, so you cannot go wrong to learn a particle here, an optative there, even a stray dual or two, even though you may not encounter them often in Biblical Greek. If you feel bad that nothing has been produced for Koine composition in the last hundred years, don't feel too bad. Only ONE book has been written PERIOD for Greek composition in the last hundred years,http://www.amazon.com/Writing-Greek-int ... 185399717X
and this too focuses on Attic. Greek comp just is not what it used to be. I have not seen this new book. No preview on Amazon is available. I would be curious to see what it is like, to see how a more modern approach to writing Greek might work, since the standard textbooks of Sedwick and North/Hilliard are so old. Not that old is bad when it comes to Greek resources. In fact, if you browse through Google books, you will find even older books than these, some designed for middle school kids, some that focus on certain books like the Anabasis, that you might find helpful.
2. Christophe Rico's Polis. Rico's book is not a composition book per se, but, in the final analysis, I do not feel comfortable recommending Sedwick or any of the other standard texts. I tried a few of these and they did not work for me. First of all, I found the process of simply translating into Greek long and often boring passes of Victorian English prose very tedious, and I did not find much step by step guiding into the process of composing. And ultimately, I think a book which uses so much English defeats the purpose of writing in Greek, which is intended to produce internalization through immersion in the target language. Rico, on the other hand, has you do some simple composition and he does not use any other language in this process. Rico, more than any other resource I used, got me started in the process of writing (and listening to, and speaking, and thinking) in Greek. His exercises on οὖν versus γάρ and the proper use of the article with demonstratives are both very basic and very helpful. Rico is still a bit expensive, but he is one of the few Greek resources I think is worth paying for.
3. Your question is somewhat similar to "Are there any good resources available for learning how to ride a bike?" It's true that not EVERY activity is best learned by just doing it, but most are. I more or less taught myself to write in Greek by just writing a bunch of it on-line. Now, I know what you are going to say. But if you think my writing is bad now, you should have seen it when I first started it. As bad as my Greek writing is, it HAS gotten better. I, and a few others, have in fact left behind an on-line paper trail to show that ones Greek composition WILL improve if you just start writing a bunch of Greek, AND YOU CONSTANTLY CHECK THE ACCURACY OF YOUR GREEK WRITING IN COMPARISON WITH READING TONS OF ACTUAL GREEK.
The best Koine composition resource I can recommend is the Textkit weather thread. Start with the simplest sentences. λάμπει ὁ ἥλιος. Shift the tenses around νομίζω ὅτι λάμψει. Experiment with word order. Then gets lots of practice with the basic post-positive connectives οὖν, γάρ, and δὲ. Experiment with particles and compare this to what you see in the text. δη is a good one to start with. γε is trickier because it is enclitic; I still don't really have a sense of how to use it. Don't worry about writing well. Worry about writing often.
And remember two things. The purpose of writing Greek is not to produce a perfect product that someone else would want to read. (μὴ γένοιτο!) The purpose is to internalize the forms, vocab and syntax in an active way that reading-only can never attain, so that your READING fluency improves And in your case, if you are truly interested in learning Koine, remember that Koine itself was a simplified, less-than-perfect language that itself suffered from "interference" from Semitic languages. John the Revelator was not afraid to write bad Greek. Neither should you. Just (continue to) do it.