You have several options actually.

First, and perhaps easiest, if you like Star Alliance switch from BWI and fly from DCA. DCA is a US focus city, and thus several cheap nonstop flights originate from there daily. You could then use your Star Alliance account to build points for Virgin (assuming they are a member or close affiliate like you said, I haven't a clue). As a newly converted UA flyer I would advise you to fly through IAD. There is simply no better carrier than UA and I can garuntee you that flying as frequently as you are once you become Premier you will not be disappointed. As per One World there are no options for NS from any D.C. airport.

Second, just join DL SkyMiles and start gaining miles there. NW, CO, and DL are all domestic members of SkyTeam, as a Gold NW flyer I won't say its terrible, but there is better. Check out the SkyTeam website, though I can assure you they are teamed with all the major hotel chains (ie- Hilton, SPG, etc). You can gain 500 miles per stay that are credited to your SkyMiles Account, then you can directly transfer the miles to the hotels, or as noted earlier send them through AmTrak.

Hope that helps...