Here is the part of the iracing.com forum post that i think helps explain what you need to do mate.
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If any of you have an existing Steam account, didn't buy iRacing through Steam, but would like to link your iRacing account to your Steam account, here's how:
1) On the iRacing member site, go to "Account/My Account". At the bottom of the page there's a button to generate a Steam Key - click it. Make a note of the key; you'll need to enter it twice in the next few steps.
2) Click the "Click here to enable it" link once you've generated your key. This will enable the Steam Overlay while running iRacing. You can always turn this off if you wish in the "Settings" panel.
3) Open Steam, and in the top-level menu, click "Games", and then click "Activate a Product on Steam".
4) Follow the steps on-screen until you're asked for a Steam Key. Type in your Steam Key, and finish the installation.
5) If you then launch iRacing via Steam, you'll be taken to the iRacing login page, where you will need to enter your Steam key again, but only once, in order to link your Steam account to your iRacing account. Now you will be able to login either directly at iRacing, as always, or by pressing "Play" from within your Steam library.
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After reading that it looks like you will need to re sub via iracing.com first then move your account over to Steam.
Jason.