Hi Roman,
Lumira is pulling this directory from the Windows' environment variables, and it seems your system admin has set it to a shared network directory. The problem of hard coding the directory as you found is that all users will use the same one. At this point Lumira cannot read %USERNAME% variable. You could add this to SAP's Idea Place for an enhancement request and hopefully see it in future releases.
Kind Regards,
Trevor Dubinsky