This is how I'd do it:
She comes in and hands you her cup as usual and says what she wants or whatever.
You go get her her drink and after handing it to her say, "What's your name" she replies with "(her name)", you say "Well (her name) I'm (your name), I get off work at (time) how about you meet me back here and we can have some coffee and get to know each other". Remember the worse thing that could happen is she says no and at least you tried. I doubt even if your boss was listening for this 15 second conversation you'd get in too much trouble. Good luck and let us know how it goes
