Erik
You can enter Vertical Angles into Slope fields already - look at your Units Settings under Project Settings to see what your Vertical Angle Settings are (Most likely set to Zenith Angles). That being the case a Slope can be entered in as z6 , z 6 or Z 6 00 00.00 for 6 degrees. If you need minutes and seconds then the format is z000 00 00.00.
Another great day for modeling
Alan