non-SQL code in RQB extras

here is the custom fields doc for relational query builder: https://orm.drizzle.team/docs/rqb#include-custom-fields
i would like to have an "extra" field in my query that is generated via plain typescript, not sql. so, something like:
    const threads = await db.query.dmThread.findMany({
      where: or(eq(dmThread.userA, locals.user.id), eq(dmThread.userB, locals.user.id)),
      extras: {
          other: locals.user.id == dmThread.userA ? dmThread.userA : dmThread.userB
      }
  });

how can i do this while still preserving typing, concise code etc. that drizzle usually provides? do i really need to iterate over all this and do Object.assign (if so, how would i still ensure the data is typed?) or is there a better way?
Drizzle ORM is a lightweight and performant TypeScript ORM with developer experience in mind.
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