Not directly, like we have for the machining data.
Since these files are basic ascii, you can use a text editor. I would organize your spreadsheet so that the fields are in the matching order, then save as a .csv file. Open that in a text editor, replace all the "," with " | ", and with a little editing, you can paste this in to the out of the box files (part_materials.dat, tool_materials.dat).
Mark is right. This is also documented in the NX online help. I mean it is documented the import only relates to feeds and speeds data. You can find an example csv in ../MACH/resources/library/feeds_and_speeds (if I remember the path correctly)
Usually it is not required to import such data, because there are not so many different ones. Let me give an example. At my company we have a large database of thousands of part materials that are used in design for weight management, environmental and law purposes. Nobody in CAM would use all these materials in the machining data logic for feeds and speeds generation. Instead our CAM departments have maybe up to 20 part materials /at the highest) that are covered. If you need to machine steel, there is not so much difference between the kinds of steel.
To sum up: You probably need more time to set up an automatic import than to enter this data manually.
With the Edit Machining Data Library command you can select an existing entry and press Insert. If I remember correctly this provides you the editor window with the previously selected entries values. You maybe have to type less if you use that mechanism.
When I went to do it I found must have started some time ago and had most of the work done just had to add a couple of new grades and append to existing files