Yes, the holy souls in Purgatory are helpless, but only with respect to their own situations. They have already been judged and cannot, through their own spiritual power, reduce their own purgation. This, however, does not mean that they cannot intercede like other souls. All soul may ask intercession for others. The ones in hell do not, as in their torment they refuse to turn to the God whose justice they have called upon themselves through their actions. We have evidence of purgatorial societies which not only pray for the souls in purgatory but also pray to them for intercession going all the way back to 336 AD (the Brotherhood of Constantinople). It is not the same as asking a Saint or even one of the unrecognized souls in heaven for intercession, but asking those holy souls in Purgatory for intercession is akin to asking your next door neighbor to pray for you, just on a much larger scale. You can only reach so many people in your life to personally ask for prayers. By praying to those in Purgatory, you have access to an untold multitude of intercessions with our Lord.
God Bless,
Br. Ben, CRM