I am not quite certain what you mean by a 'source.' Let's take for example a U.S. Census page from 1940 that details the names, ages, sex, and other information for a family. The source would be the page. FamilySearch, MyHeritage, and Ancestry might have copies of that page and references to the source but the source is the page itself. Using a FS reference as a source is commonly done and acceptable as it does contain the actual source although one could forgo the indirect reference.
As WikiTree can host images of documents, then a reference to an image on WikiTree that is an acceptable source would be valid for use in FS or anywhere else. Note, it is not the profile of the person rather the image.
Another aspect of 'source' is really just a reference in the form of a web link. As Dennis Wheeler has already pointed out, this is done by Justin York's 'Connections App' or one can do it by hand. I do this myself religiously so that there is both a link from WikiTree to FS and from FS to WikiTree creating a doubly-linked pair. I also sometimes add a web link from my Ancestry tree profiles to the corresponding WikiTree profile (but not the other direction, of course).