Parts of this question may be linked to I have Multiple Subprocess files in test case, would like have Property Set in Subprocess Path so might be best to watch both threads.
Current sub-process input and output parameters are statically defined inside the actual test case asset at rest on the file system. Perhaps best to raise this question as an Idea so the CA product team can determine the usefulness and impact.
I suppose you could construct a single value (let's say a String) and pass that value to each subprocess. I do not recall if DevTest allows passing an object between test cases and subprocesses or just a String.
Let the main test case setup up a property. Add script to build your dynamic key/value pairs for a given subprocess. For example:
sub1 subProcessVals = "prop1=someVal&prop2=valA&propB=anotherValue"
sub2 subProcessVals = "propX=someVal&prop2=valZ&propY=yetAnotherValue"
sub3 subProcessVals = "prop1=someVal"
Let the Subprocess accept the input request argument (subProcessVals), add a script step to split (&) and parse (=) the key/value pairs into TestExec properties. I would also add logic to ensure all the expected properties were contained in the respective test case.
Although doable, I think you will find the above approach problematic when adding new sub-processes and testing / debugging due to the complexity.