I need to look at the code but I suspect the Archives plugin is not respecting the flag to tell it not to prompt. Should be able to fix it, in any case.
In a short test case, this seems to just copy MyFile.ext as itself to the destination. Even if I've already created a MyRArFile.rar to "catch" it, MyFile.ext just ends up as a sibling alongside MyRArFile.rar, not merged into it.
Pulling back, I'm trying to do this as part of a script, where I'm using a foreach loop to make the .RAr if it doesn't already exist or add to it if it does on each loop. That way, if I don't enter the loop, I don't have an extraneous .RAr file I created at the beginning. (Though I guess I could put some conditions on creating it — or delete it at the end if it's unnecessary.)