Premiere CC 2015 doesn't support 18fps natively (not in sequence, not in export)
In fact, it's not entirely true. It seems not documented, but Premiere CC 2015.2 can in fact work with a 18fps sequence.
To create a 18fps sequence, follow these 2 steps :
1 Clip > Right click > Modify > Interpret > Frame rate > Assume 18fps
2 Clip > Right click > New Sequence From Clip
That's it. You have a 18fps sequence, that you can then export properly using Voukoder
The only drawback is that you must not edit the sequence settings afterward, because it would reset the frame rate to 10fps by default