There may be an easier way than this, but this is what I do. Open one part in the library and add the property field. Save the part and library. Under tools select Export Properties and select OK. Open the *.exp file in an editor such as Excel. Find the column for the new field and use copy and paste to insert say a "_" into all the rows for that column. Save the file. Under tools select Import Properties. All your parts should now have the new field with the "_". I tried using just a blank or space in the field, but it did not work.
Have a good day,
Capture 16.5 S022, Layout 16.2.0p001, PCB Editor 16.5 S025