When talking about TestManager scripts what are you referring to? Is it manual test cases (steps and such) or is it a Robot script being converted to WinRunner or QTP? A little more clarification so that 'we' can help.
Bottom line though, there isn't much, or at all, conversion tools for either (manual and automated scripts).
You will have a manual effort to convert. You didn't think they would make it that easy did you?
Well... I guess if you had the time/knowledge to do it you could write the SQL code to pull the information from the Rational TestManager dB and use the Mercury OTA to push it into TD/QC dB.
You have to know both tools API's and database structures. Do something like pull the test case data from TestManager into a CSV file and then use the Mercury Excel Import tool to pull it all in from the CSV (or use the OTA API).