Hi @suenugus,
Not as of yet - there is some work being done for this in future versions of OJS: Improve UI to add variable data to emails · Issue #7743 · pkp/pkp-lib · GitHub
But, if a variable is not used as part of the default template, you likely cannot just insert it in any template. Please see here for a more detailed explanation: Can prepared emails get the section the submission was sent to? - #24 by rcgillis
Best regards,
Roger
PKP Team