Create (generate) new Storyline variable with Javascript (or by any other method!)

Hello,

I fear it's impossible but I'd like to create a variable in Storyline using Javascript (or whatever trick that works) because variables are NOT saved in themes.

Don't know if this can help but I noticed that you get a variable automatically created in a new project if you copy a text box containing a variable from another project (with %myVar% in text box for example).

Thanks in advance.