is it possible to convert to jmx the http test scenarios prepared in RPT, and exported to some intermediate fomat, by using some free tool?
There is no obvious RPT to JMeter converter, however you can get at least test "skeleton" using JMeter's recording functionality.
Configure JMeter's HTTP(S) Test Script Recorder and start JMeter proxy. The easiest way to get "good" working configuration is using JMeter Template feature
Configure RPT to use host running JMeter as a proxy. In order to do so: