Debbie,
Absolutely! There is no exercise done in SJP with the barbell that can't be done easily with barbells, just stick to a weight that works for you. For example, Cathe uses a 40lb. barbell for squats and you could sub with a 20 in each hand, or lighter depending on your level.
As far as being "ready" for Step Blast, don't worry about it. It is an awesome and FUN workout that you can ease into. Just start with the warm-up and do what you can from there learning each combo as you go. The best part about the DVD of these two workouts is the HUGE number of premixes for each video and between videos. You can break down, add on, shorten, lengthen, etc. to your premixin' heart's content.
Have fun!
Mattea