No, that option does not exist yet. (Not even in the manual  )
One way to work around this for now is to set the lower limit for the 'visible range' (Tools/Options/View) to the height you want to default the track to.

Another would be to just draw at 0, and elevate all the connected track later.

David.

In fact, it is not really possible to fix a point's height (yet).

If one creates a grade, all neighboring parts move up (or down) as much as needed to satisfy the given descent percentage.
If one levels, all connected track track is moved up or down to the same height.

So: the easiest way to work with two levels is to first disconnect the piece of track that will become the gradient. Next, move the rest of the track to the desired levels.
Subsequently, create the gradient, and reconnect all the track.

I hope that the attached picture clarifies the process.
Of course, in any of the steps, the undo/redo functionality of the program can be a great help.

David.

Well, it's just easier to demonstrate it when moving it away. But also, it's easier to click the right endpoints to set the height, and keep track of what you're doing.

Good luck!

David.