I do not know if add vst plug in button is available in MMM 17 or not because in MMM 16 it was not available, however in the MMM 16 Premium version it is available. But in my experience of MMM 16 I will share what to do.

You wrote:

Is there a way to add synths to Magix 17?

By this question do you want to add, additional synth you have downloaded from third party? or you want to use synth from Music Maker.

Case 1. From third party:

Locate the synth folder of Music maker 17, and just copy or drag and drop downloaded synth (with dll extension) inside the folder. Restart Music Maker. It should be added in the program. But some synth may not work in Music Maker.

Case 2. Synths from Music Maker.

Simply when you install MMM 17 synthesizers should be installed together other stuff and when you will open Music Maker 17 synths should be added in the list. 

Hope this answer might help you.

nihon94