David Neill has a nifty post over on civil3dpedia regarding the addition of custom variables within a custom part. Nifty, in that I guess if you need this, here it is.

Civil 3D Part Builder – Not JUST 3D – Part 2 | EE Civil3Dpedia.

A few things to remember for all of you that just think you have to dig into partbuilder:

1. 95% of the display issues I’ve EVER seen can be handled with styles. Perhaps a little drafting on top.
2. Custom parts will not go in and out of StormSewers in any way shape or form.
3. Custom parts are notoriously difficult to share internally, and when you send them out, you’ll have issues sharing there as well.

None of this should be to imply that you can’t and won’t have success with PB. But really, unless you abso-fricking-lutely have to, I wouldn’t suggest you dance in that mine field.