I would take a look at existing (example) stock parts and ptb that are supplied with NX. For example, the steel_pipe_mm.prt and corresponding steel_pipe_mm.ptb. Open the prt file and see how it's been qualified and then open the Part Family spreadsheet. Note that it only has 3 columns; Part_Name, wall_thk and pipeod. Now open the ptb in a text editor and note the similar columns; PART_NAME, WALL_THK and PIPEOD, along with several others, one of which determines the Part Family Member to be used for your specific design (which OD and Wall Thickness).
NX supplies utilities that create the PTB's (ss2ptb) and associate them with their corresponding prt files (update_fam_from_ptb). Using an exising ptb file as an example you can create your own ptb to be associated with your new stock prt file. Once these are complete you will need to add your stock as a new node in your Part Library View file such that it will appear under your Steelwork node when adding stock.
Hope that helps,
Ben