Not all of the library parameters are retrieved and stored with the NX tool object, because they are not needed to generate a tool path. This includes parameters like you show here - an additional parameter that has been added to your system by customizing the tool library interface.
Even though they are not saved with the NX tool, these extra parameters can be read directly from the library by postprocessors and shop docs. This may be enough for your requirements. What do you want to use the parameter for?
If you really need the parameter saved with the tool, and in the tool dialog, then you can add it as a User Tool Parameter. Take a look at the file mach\resource\library\tool\ascii\dbc_tool_ud_data_sample.tcl. It has samples of hard coded parameters, and parameters read from the tool database. There is documentation here.