Only way I can think of is to copy the ARD database onto the Windows box and point WHD to it.
Then just periodically copy it over to keep it up to date
I'm currently working on setting this up, first hurtle was setting up ARD. I've not used it all that much so it's new territory to me. Below is the outcome of 4 hours of troubleshooting and phone time with Apple support. This is also my plan, I've not got it all set up but in hopes of saving someone some time I'm posting now and will provide an update later:
Step 1 Setup ARD: My issue was getting the entire IP ranged added vs the scanned range which only shows IP's/devices it's found while scanning, thus only adding this limited number of devices to the "system overview" report needed for WHD. ARD has an option to import IP ranges from a txt or csv file; however, I was running into an issue when going to file import the txt or csv files were grayed out, the trick is to create a new Scanner and change the option to File Import and then select the file.
Now you can select all IP's and add them to the System Overview report. By having all IP's in there regardless of there not being a device currently tied to it is if there ever is a device then you don't have to rebuild your report, which would be a pain with 20+ subnets.
Step 2 Transfer SQL file to Windows WHD server: Set up Automator (How to Schedule Automator Tasks in Mac OS X | Chron.com ) to copy the /var/db/RemoteManagement/RMDB/rmdb.sqlite3 file from my ARD server at a specific building to my WHD server and name the file "buildingname.sqlite3"
Step 3: Set up WHD Discovery connections for each building filename
I'll post an update once I've got this all going.