It is annoying that the template doesn't resolve the symbol at template caller.

[sls1005]

You need to give it the symbol (by passing it as an argument).
    
    
    template t2() =
       template t1(r: untyped) =
          r.add("hello")
       var res = ""
       t1(res)
       echo res
    
    proc p1() =
       t2()
    
    when isMainModule:
       p1()
    
    
    Run