from another thread:
Posted Today at 10:43 AM by mm2270
This is totally possible to do now in version 9. It wasn't so easy in versions 8.x though.
If you are using version 9.x, go into Management Settings > Extension Attributes. Create a new EA called "Retired Asset" or whatever you want to call it. Set Data Type to "String", Inventory Display to "Purchasing" and Input Type to "Text Field" Save.
Now, back in Inventory., locate the retired Mac, go into its details and click on Purchasing finally click the "Edit" button. Scroll down and your new EA should show up there with a field where you can enter whatever you want in it (like the Asset tag #) and save it.
Does that help you at all?