I was poking around in the sldmat file in 2005. This is what I found:
The capital letters such as EX, NUXY, etc. refer to standard material property nomeclature in Cosmos/M. If you can get a hold of the Cosmos/M documentation you will find all of the material property designators. This documentation can typically be downloaded from SRAC.com. Whether CosmosWorks will support a particular designator is something you will have to determine by reading the CW docs or by testing. The units used above are MKS units so IPS units will have to be converted.
The sldmat file itself appears to be in XML format. It resides in \lang\english\sldmaterials with the sldmat extension. Any file with the sldmat extension will appear in the materials list box if it is in that directory. TOOLS/OPTIONS/SYSTEM/FILE LOCATIONS/MATERIALS can be pointed to multiple directories with sldmat databases as an alternative to the above directory.
My guess is that the tag and the tag are all that are needed to add a material to SW. The tag has two parameters, name="" and matid = "". It would appear that the matid must be sequential.
I don't know if Access or Excel have the ability to read in an XML file and make sense of it, but if they did this would be a convenient way to manage your properties.
This API example could also be useful: Get Material Database and XML Schema File Names Example (VB)