If it is for testing, as you say, why don't you just try it and tell us how it works?
I have no idea why one wouldn't want
because this is the easiest and fastest way to do this. Arguably, using NIM takes some preparation, but
mksysb takes next to no preparation at all and will produce fast, reliable and "well-formed" results.
Your proposed solution, even if it works, will gain you nothing in terms of time to do it or reliability/desirability of the results. So i suggest to think it over. As long you do not present a really convincing argument (which i, even racking my brain, am not able to conceive) for why you want to forego some supported IBM ways in favour of something which might "also work" at best i suppose your logic to be flawed from the beginning.
I hope this helps.
bakunin