Replace the elbow-piece -- it's really a type of conduit body called a LB (L for elbow, B for out the back) -- with a box and make the connections to the new wiring there. You may have to pull a new stub-length of THHN to the existing receptacles to get enough slack in the box on the existing incoming wires to make the junction.
As to the new run -- I'd use two lengths of conduit for the exposed run: one from the tap box to a box mid-span that serves as a tap point (I'd use a T body, but there's not enough space for it unless the conduit run is offset sideways from the switch location anyway) and the other from the upper box (it can be a handy-box as the box fill's minimal) to the attic, then use a nipple to stub off from the back of said handy-box to the box with the switch in it (which'll need to be capable of taking a conduit). A one-gang box should be more than adequate for the switch box, by the way. The wire would be THHN to match the existing circuit gauge, in standard colors (black/white) of course, with only hot (black) wires run to the switch location from the tap box behind it.