How to make Ease-In and Ease-Out for slow motion videos in new iMovie?

If I record slomo video with 120fps on iPhone 5s I can choose the start end end point of the slow motion in the camera roll and the effekt is not instant, it eases into slowmotion and eases out to normal speed - looks very good that way.

But as we all know there is a lack of saving the rendered video in the camera roll and send it e.g. via whatsapp, only sharing option is mailing, youtube etc.

So I was happy to see that the new iMovie supports the 120fps slowmotion videos and you can change the tempo. You can do it for the entire videoclip or you just cut the video into parts and make just one part slow... but there is only a hard transition between normal and slow speed, no ease in oder ease out, you can not edit this.

Is there any possibility to do this? I do not want any transition effect like fadeout or curtain etc.

Thanks for any help.

B.