there is no direct method to do this, however you can use QTP AOM to achieve this. e.g
for more detail, search for "ActionParameterDefinitions" in qtp help.
Code:
Test.Actions.Item(1).ActionParameterDefinitions.Add "InParam", "Param1", 0, 0, "Input Value"
for more detail, search for "ActionParameterDefinitions" in qtp help.