There are a few different pieces to what you are asking and I hope the following references/answers help you out.
First, if you want to add the source files from Prj_B to Prj_A, then you can either link files and/or folders from Prj_B to Prj_A. See http://processors.wiki.ti.com/index.php/Projects_and_Build_Handbook_for_CCS#Adding_or_linking_source_files_and_folders_to_project
If you only want to reference the header files from Prj_B and not use any of the source files, then you can simply add the path to the header file in the compiler include options for Prj_A. It appears that you already tried that but I think you may have tripped up here:
[quote user="Toby Mole"]basically I have tried adding it to Prj_A as I would any other include directory, adding a resource path variable, using that to add it to the C2000 compiler include search path and also the linker directory search path.[/quote]
If you are using a variable, it needs to be a Build Variable and not a Linked Resource path variable - the difference between the two is explained here:
http://processors.wiki.ti.com/index.php/Portable_Projects#Portable_Projects_in_CCSv5
At first, I would recommend starting simple and get it working by specifying absolute paths in the compiler include search path option to make sure it is finding the header file. Then you can make the project more portable by creating a build variable and changing the include search path to use the build variable instead of the absolute path, as per the above wiki link.