Unfortunately Ian, they're not quite the same... some slight impedance mismatches cause issues... (this is why when i made my in-ear headset, i needed earphones with an inline volume control and have it set to minimum, otherwise they are too loud)
That and headset mic's generally need more power than a PC soundcard will provide...
I actually modified the adapter box that I built to use USB power instead of the 9V battery

Seems to work OK...
The hardest part was sourcing the headset jack... they're an 'odd' size... but I managed to get one from
RS Components...
You can of course buy adapter boxes... but they're like US$40 + shipping etc... not sure of any local retailers...
Send me a PM if you want any more info...