How to connect a 3.5mm microphone (not amplified) to an iMac for recording?

I have a headset which has separate headphone and microphone jacks (both 3.5mm) that I would like to do some recording with.

I initially thought, just plug headphone into headphone and micrphone into line-in and it would all work. Needless to say, I didn't get any sound from the microphone. After reasearching, I discovered that the line-in is for amplified sources only, not basic microphones and that the mic needs to be ran through an amplifier first. Alternatively, I could get an adaptor which combines the 2 jacks into 1 and plug it into the headphone plug as I understand that it can pick up mic inputs.

I don't have any adaptors and in the past (on PC) I've been able to run a mic through a line-in and use software to amplify and reduce noise to get a mic to work and record.

I found software called 'LineIn' but that doesn't seem to do the job. I was wondering if there is a software approach I can use which will amplify the mic on the line-in so I can use it to record and thus, not have to buy yet another adaptor?

My other thought (which I have known to work on a PC); Would it be possible to swap the line-in to be used as a headphone output and then use the headphone ouput as a mic-in?

Thanks!

What you need is a audio mixer (I personally use and like the Behringer line)

The inputs/outputs built in to computers (both Macs and PCs) are basically rubbish - they are looking to give you functionality to mark a check box for marketing purposes, not give you the ability to make good to professional quality recordings..

This mixer will give you a USB interface to your Mac while allowing you to bring two sources in while having granular control over input sound levels.

Granted, it's not a "free" solution, but at $30 USD, it's a very economical solution that gives you better quality and more flexibility than what you have at present.