These are two questions (at least):).
1. Yes map the ramp path and give it highway=footway, wheelchair=yes or if it is ONLY for wheelchairs highway=path, wheelchair=designated, foot=no, horse=no, bicycle=no
2. Yes in the case described you just connect to the road line (or you can add footways along the road if you know where is it).it).
It is also useful to tag the incline, to specify which way the ramp goes. At least incline=up or incline=down (relative to the direction of the way). Or if you are able to measure the steepness, you can tag that as a percentage or angle. See http://wiki.openstreetmap.org/wiki/Key:incline