I've loaded a couple documents I can't find on FamilySearch to the Memories tab, like a birth, marriage, or death record. Is it possible to also link these documents to the Sources tab? If it is possible to do this, how can I go about doing it?
Yes, by creating a source.
You may wish to start by copying the URL.
Then go to the source tab of the individual, you can click 'Add Source', and choose 'Add New Memory Source' and fill in the information about the source , Date, title, and URL, then click on the "Add a Memory" and choose from the Gallery, to attach the memory to the list of sources. Be sure to click Save at the end.
We wish you success in adding the memory to the source list.