The general requirement is to have a minimum number of hours on type to convert your license. Usually, this is a minimum of 500 hours on type and 1,500hrs total time, but some airlines seem to be permitted to hire with as little as 300 hours on type.
Indeed, for the ATR, at least one operator requires specific experience on the 72, 42 experience doesn't count!
Non type rated entrants usually have to go through the airlines specific cadet entry program, be trained on certified simulators with state licensed instructors and examiners. If you have no hours on type, then this is an option. If you already have a rating with no hours then it is conceivable that you maybe credited some reduction on the full program at somebodies discretion.