There do exist in-line booster fans that can be used with clothes dryers for long duct runs. But you cannot use just any fan for this purpose.
One proper dryer booster fan is available from a company called Continental Fan. Continental's DVK series of fans can be purchased with various different controls to assure they turn on and off automatically with the operations of the clothes dryer. Quality is important for a clothes dryer booster fan to keep it running and keep it running safely.
Other fans specifically labeled for use with clothes dryers are supposed to be showing up on the market shortly. Check the label of the fan you are considering buying to be sure that the manufacturer approves its use with a clothes dryer.
There exists an air flow alarm that can tell you when your dryer duct is restricted by lint build-up anywhere in the system -- you will find Lint Alert details here.