Adding Admin for Secondary Site

Hello:

I am the administrator for one site “A” that oversees a number of other small sites “B, C, D”. Each of the smaller sites “B, C, D” have their own individual administrators/editors. However, one of the smaller sites “C” was down for a while and the logins for that smaller site has been lost due to new people taking over the position. Thus, I am unable to access and edit site “C” without creating a new user, but the only way I could do it is through the main site “A”. It appears that all I can do is add new users that can edit site “A, ” rather than stipulating that the new user will have control over site “C” only.

Is it possible to create an administrator for site “C” from my position on site “A”? If so, how should I go about doing it? It is pertinent that the administrator of site “C” only has access to site “C,” due to data collection and other possible security issues.

Thank you very much for any and all suggestions.