You can also do the below instead of using dynamic property.
1. Create a ActiveX script before the execute process task
2. Use the below code to change the parameter
Dim oPackage
Dim oCustomTask
Set oPackage = DTSGlobalVariables.Parent
Set oCustomTask = oPackage.Tasks("Execute Task Name").CustomTask
oCustomTask.ProcessCommandLine = "Command" + " My Param"
3. On sucess of Activex script, go to the execute process task.