You load an HDR the same way you do a bitmap. You raise it the same way, through the color picker. As for F-stops, keep in mind that it is exponential.
+1 stop = x2
+2 stops = x4
+3 stops = x8
+4 stops = x16
…
-1 stop = 0.5
-2 stops = 0.25
-3 stops = 0.125
…