I personally always feel comfortable adding and connecting additional family members to profiles other people manage... indeed, you could say that is what Wikitree is all about!
The exception might be if your conclusions about someone's family members apparently contradict what is indicated in the profile - in which case, it's good practice to first reach out to the profile manager and explain what you think and why, using sources. But if looking at the profile you can reasonably conclude you're on the same page, connect away!