If you don't feel she is ready for a toddler bed or "big girl bed" yet, have you tried a crib tent? You can find them at Babies R Us or maybe even Target or Walmart (not sure). They attach to the crib and prevent kiddos from crawling out.
We've recently moved our 2 year old (26 months) to a twin sized bed which is up against a corner so that the head of the bed and one side are touching the wall. Then we have a short bedside rail on the foot of the bed and an extra tall, extra long rail along the open long side. Our daughter moves around a lot, and she is sleeping better now that she has more room to move and she has only crawled out of bed a few times - but it's safer than falling out of a crib.
Try the crib tent (I have heard great things about it) and then as baby arrives (congrats) you may transition your toddler to a toddler or big girl bed during the time your newborn is sleeping with you all in your room (a few months at least).
Good luck and congrats on the new baby.