It's easy to mix a juice down in mg....but to raise it is problematical. I wouldn't recommend just adding higher strength nic juice to a flavored juice as you would be diluting the flavor.
There is always the swap thread here: http://www.e-cigarette-forum.com/forum/vapor4life/85254-v4l-swap-thread.html
There is always the swap thread here: http://www.e-cigarette-forum.com/forum/vapor4life/85254-v4l-swap-thread.html
