You can use Imap.SendCommand method, after you authenticate with admin credentials:
imap.UseBestLogin("admin", "admin-password");
ImapResponse response = imap.SendCommand("PROXYAUTH user");
if (response.Status != ImapResponseStatus.Positive)
throw new Exception("Failure: " + response.Message);
Next release of Mail.dll will include this as a regular method on Imap client:
imap.UseBestLogin("admin", "admin-password");
imap.ProxyAuth("user");