If your ultimate goal is to get the bees into a top bar hive from a Langstroth Nuc or Langstroth hive, you can cut the bottom and sides of each langstroth frame off and using the falseback as a guide cut through the comb to make it the same angle as the top bar hive.
You will need to cut off the ends of each Langstroth frame as it is normaly longer than the top bar length.
Langstroth frames are normally 19' in length. Our Golden Mean top bars are 18” in length. So transferring the Langstroth frames into a Golden Mean hive can work. (For the Original BackyardHive 14 1/2" top bars it would be tricky as that is a lot of comb to have to cut off the Langstroth frames.)
Now that you have only the top of the frame Lang , put this makeshift "top bar" into the hive and secure the comb with some sort of thin wire wrapped around both comb and top-bar to keep it in place. You could do this toward the front of the hive and then as the bees start using the top bars, in the next year or two you can start to circulate these makeshift bars out and replace them with real top bars.
Here is a video on Vimeo about transferring a langstroth nuc to a top bar hive
If the bees are in a full Langstroth hive (not a nuc), another possibility would be to keep the bees in the lang hive without using foundation so that the bees have the freedom to draw out their own comb and then next year get a swarm and put it into a top bar hive. If you do decide to use this method, be sure to give the bees some sort of guide for where to start their comb on the empty frames. Here is a link for more information about foundationless frames: http://www.bushfarms.com/beesfoundationless.htm