Hi Essi,
welcome to the Zeebe community 
I’ve read through the Feel documentation particularly around expressions but the editor doesn’t allow spaces?
Good point. The zeebe-modeler plan to improve the UX for expressions. However, feel free to create an issue here: Issues · zeebe-io/zeebe-modeler · GitHub.
Yes. That should be possible in general. You could have an input mapping with a source expression
=if (new_variable = null) then 1 else new_variable + 1
and target new_variable.
Note that the variable name should not include an operator like -. Instead, it is recommended to use snake_case or camelCase.
Also, you need to ensure that the variable is present (i.e. initialized with null) before. Otherwise, we could lead to an unexpected behavior (A missing variable/nested property can not be handled · Issue #4212 · camunda/zeebe · GitHub).
Does this help you?
Do you want to discuss more about the context where the variable is used?
Best regards,
Philipp