Dynamically Creating Rule Inputs

When we use Builders in interface , It will create Interface with rule inputs of given CDT or RecordType , I'm really curious to know how they creating variables dynamically(My guess is that they are running some java code in background) . As far as i know we can't create rule inputs dynamically ,If not correct me. Is there a way to create rule inputs of desired type dynamically??. I created a replica of Builders using functions like keys and runtimetypeof(), But i used local variables in value and saveinto parameters. I want to replace it with proper rule inputs.