I have been putting down cash, travellers checks, Disney Dollars, or Disney Gift Card to enable room charging for several years now.
As CDSTapisRouge said, not every CM knows how to do this; in fact, many don't even know that it can be done or think that it can't. I often have to ask them to call their manager over to explain it to them, but it doesn't take long and it's apparently not difficult to do once it's explained.
I'm used to that, however. In the 2 years or so that I've been getting Disney Gift Cards to save for my trips, I have encountered only ONE CM at any of the Disney Stores where I go who knows how to add money to an already-activated card without calling the manager over.
"Press the green button, then hit 1, then swipe the card..."