On many of the lists in my farm, there are custom columns based on the needs of the group that is using the list. Every once in a while, we need to add a new content type to an existing list and want it to make use of those existing custom columns. When I first encountered this need, my search results were pretty fruitless and most of what I found said it could not be done. In fact, it can be done so I thought I'd share how I did it in case anyone else finds themselves in this position.

1. In the list where you have your custom columns, go ahead and add the new content type that you were wanting to add. In my case, I wanted to add the "Link to a Document" content type and wanted it to make use of the "Category" custom field that had been created in that list.
2. Go into List Settings (or Document Library Settings if you are adding a content type to a document library instead of a list)
3. Click on the content type you wish to add the custom column(s) to
4. Click the link for Add from existing site or list columns
5. Be sure that List Columns is showing in the drop down under Select columns from: and move the columns you want to add from the left side to the right side and click the OK button

That's it! I know this seems a bit simplistic but I think that what caught me and probably caught those who said this couldn't be done thought it wasn't working because I noticed that the custom columns I wanted to use wouldn't show up on the form until I went to check the item in. At first, when I clicked on new Link to a Document, I just got the name & URL boxes. But, once I filled them out and clicked OK, the check-in form appeared containing my custom columns.