Its a bit convoluted, but I believe this will get you there.
1.)In the drawing that has the Desc Key you want to copy, in the Toolspace Settings tab, right-click the name of the of the drawing. Choose Edit LandXML Settings.
2.) In the Export tab expand Point Export Settings turn Export Description Keys to ON.
3.) In the General pull-down menu choose Export LandXML. I exported only the Point Groups (unless there's more that you want to add to the DWT file that is) using the filter method.
4.) Open the .DWT file that you need the Desc Keys in. Under the General pull-down menu choose Import XML. Import the CGPoints.
5.)You will need to delete the points that come in (as well as the Point Groups in the Prospector), but if you check the Settings under Points that Desc Key file will be in the DWT file. Save it and you should be able to use it from there on out.
HTH
Russ