Joe:
I finally go around to trying something. Here is what I came up with. The HELP (may it rest in peace) says that we cannot add a size but we can change an existing one. I can only assume you can do this since I am using 2014.
1. I opened up the Catalog Editor (You will find this within the START menu under Programs-> Autodesk->Autocad Civil 2013-> Content Catlog Editor.
2. I used Flanged Piping and picked out a size that I did not need. Changed all the required information
(I.E., inside diam, wall thickness, outside diam, etc. and made a 54" Pressure Pipe like you wanted.
You need to set the Catalog Current (the one we used above.)
I drew a profile for ONE PIPE, NO Fittings. Added a second pipe and it seemed to work. Obviously, the fittings must be changed if you need some.
Bill