If it is not indexed on Ancestry.com you can't do an automated attach-to tree.
Material posted on findagrave may be copyrighted: photos taken since 1928, material written by the creator of a memorial or other contributor, etc. Copyrighted material may not be uploaded to your tree without permission of the person who owns the copyright. It is not necessary for there to be a formal copyright notice posted there; the creator of the item holds the copyright whether or not there is such a notice present.
In each MemberTree person detail page, you can create an event (say, burial). If you do it from the page with the timeline, click on Edit this Person, click on Facts and Events tab, and on the same line as Burial at far right is "More Options" button. Click that to get the create-citation option and follow the prompts to create a citation to the findagrave item.
For any file uploaded to a person page, after the file is uploaded there is an option at the right sidebar to create a citation for it.
A side note, though -- a huge proportion of material in findagrave is not related to the burial or gravestone.
There may be fantasy genealogical accounts, such as one I ran across where the person who created the memorial had no idea where the person was buried but had him attached to a cemetery (created 70 years after his person's death) that had unrelated person by the same surname; he then gave a long ditty regarding speculative genealogy for the person.
There is lots of stuff copied from an obituary or death record that is not cited, incorrect readings of gravestones, and incorrect transfer of data from death records (say, died 1949 when actually died 1943, one I ran across today) etc.
One of the findagrave organizers gave a wrong transcription of an extensive gravestone inscription for a Revolutionary War Veteran; when challenged, he refused to acknowledge any problem with what he wrote, and declined to change it.
In addition, many gravestones give actually wrong dates (such as for one of my ancestors 1849-1904 the gravestone states 1850-1905).
Also some contributors get names of cemeteries wrong, put in wrong places, and do just about anything else that can possibly go wrong.
Sorry to go on so long about this, but when creating a source citation it is wise to have in mind just what you want to use the purported cemetery material *for*. Personally I often limit references to findagrave to web-links at lower right of the person overview page, and try to find other sources for evidence of vital dates.