Thanks for getting in touch.
If you copy over the "iTunes Library.itl" and your local iTunes Media from your old iMac, then you should be able to simply plug in your external drive to the new iMac and iTunes should recognize everything properly.
To keep all the TuneSpan information in tact (your preferences and what has been spanned), you'll need to copy over the TuneSpan container in your user library.
The User Library folder is hidden on 10.7, to access it, open the Go menu in Finder and hold down the Option key, you'll see the Library menu item appear, click it.
Within the Library folder, go into the Container folder and locate the folder called "com.randomapplications.TuneSpan" and copy that folder to the same location on the new iMac.
That should be it. If you run into any problems, please let me know.
-- I edited this answer after realizing you had not yet migrated to the new iMac --
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