Hi @nadwue,
the official BPMN Standard says the following:
A Pool is a square-cornered rectangle that MUST be drawn with a solid single line (see Figure 9.2). The label for the Pool MAY be placed in any location and direction within the Pool, but MUST be separated from the contents of the Pool by a single line.
A Lane is a sub-partition within a Process (often within a Pool) and will extend the entire length of the Process level, either vertically (see Figure 10.122) or horizontally (see Figure 10.123).
So to answer your question: Yes, modelling vertical processes is part of the standard. Processes can be modelled in whatever direction you prefer.
But for the readers of a process it is in general the most natural feeling if they can read the process from left to right (like reading a text).
I am not involved in the process of developing the Camunda Modeler but i highly guess that the reason to only model horizontal processes is because this is the easiest way to read a process and thus the most used UseCase.
Unfortunately I have to say that extending the Modeler with the funcionality for vertical processes is indeed a little bit of work and can not be done with editing a few xml-elements.
Hope that clarifies this topic a little more.
Regards
Michael