What about tasks that are too simple to be worth considering in a language? In almost any machine/assembly language, "address of a variable" is just an addressing mode, only a part of a single instruction. Further, it depends on where you chose to put the variable, which may be arbitrary. For instance, for the 68000, addressing a register variable is just e.g. D1. Or the variable could be in random memory, on a heap or stack, etc. Basically, any memory address or register that the program can read and write could hold a variable.
"... To mark a task as such, add, ..."
Ok, add it where?
What prompts this is task "Add a variable to a class instance at runtime", which is meaningless in languages without built-in classes, as the solution would depend on the actual implementation of classes, which is unspecified.