Absolutely. The citation for a document you have in your personal possession is similar to one that you would use for a reference link, except in this case you are the link. You state the title of the document, indicate the source (you), indicate where you got the document from and do a transcription of the details of the document. By your citation I should be able to go to the provider of that document and look it up myself.
Something like this;
"Birth Certificate", Source: Your Wikitree ID, Personal Family Collection, Provided by: AnyState Vital Statistics, Anystate, USA, Transcription: Name: Joe Smith, Date of Birth: 12 Jun 1789, Mother: Sarah Smith, Father: John Smith, Place of Birth: Anytown, AnyState, United States, Certificate #12-234
You could also take a picture of the document you have in your possession and add the image to the profile.