The combination will not work. According to the memory matrix, it will support 24gb, 32gb, 48gb and 64gb. With your suggested addition of 2 x 8gb RDIMM's, would put it at 40gb, which is not supported. Your best bet for an upgrade would be to add 2 additional 4gb RDIMM's for 32gb of RAM on your server and set it Optimizer mode in the F2 bios.
Regards,