I beleive you can only export it manually at this time via Custom Property Editor within Orion. You could probably do something custom with SQL and exporting the table on a recurring basis.
Another solution could be to use the wget utility to retrieve the report in the Excel format using the Dataformat parameter in the URL of the Report. e.g:
wget.exe -p "http://ORIONSERVER/NetPerfMon/Report.asp?Report=Last+Boot+Time+for+each+Node&Dataformat=xls&AccountID=USERNAME&Password=PWD" -O C:\MyReport-DATE.xls
Open your web console and display the report that shows the Custom Node Properties (Create One if you do not have one yet) to get its URL.
USERNAME is the Account you type in the webconsole to authenticate and PWD the password associated.
You can probably use the environment variable %DATE% using a script.
Then you only need to schedule that command or the .bat that contains it to run it regularly.
I am not sure which other formats are supported with the Dataformat parameter.
I am getting an error when trying to export custom properties. I can not export any of them.