Do you move the resource you already have, or does it come from the bank?

It's not a battery. It would come from the bank. Similar to placing one damage on a battleground.

Does it say take one of your resources and place it on this card? No.
The resource comes from the bank.

At least that is the way I read it.

it comes from the bank.

Its great since its hard to mess with, and if you have a slow deck as it is might as well do it. Its not damage, so none of the cards can mess with it. Unless they get a way to discard a 1 support (which is somewhat more common but still a bit rare to see) they cant prevent you from taking those 4-6 resources stacked on there.