My watchlist consists only of profiles for which I am on the trusted list. I have not discovered any other way to add a profile to my watchlist. In practice, this means that if my 9th cousin once removed doesn't want to "trust" me on the profile of my 7-greats grandmother, I can't watchlist that ancestor's profile unless I want to engage in an email discussion/dispute about policies and protocols (something I generally don't want to bother with). This also means that if I adopt an orphan profile, it's on my watchlist, even if I'm not "most interested" in it.