is it possible to tune synplant to 432 hertz?

Yes and no. There is no built in support for it in Synplant. What you can do is tune each patch by moving the tuning slider. You would want to hit around -31.8 ct. Closest you get by moving the slider (holding shift down) is -32 ct, but it might also be possible to fine-tune it from your host by entering an exact amount into the parameter automation.

Another approach is to "Copy Patch" from inside Synplant. Paste it into a text editor and then edit the tuning line to:

Tuning: -31.7667 ct 

Then copy the whole text and paste it back into Synplant.