I don't think you can convert WebDriver test in to JMeters, at least not at this point.
One way is to created JMeter test plan directly using JMeter. But if you are new to JMeter then you can use
BadBoy to record your tests. You can compare its recorder with Selenium IDE.
Once you have recorded your tests in BadBoy then you can
export them in jmx file, which could be used in JMeter to create performance test.
I remember there is way to record test from JMeter also but have not used it my self.
~ seleniumtests.com