Can't handle single-user key in multiuser mode

Hello.

I have:
1. Installed ZeroNet
2. Registered at ZeroID (then got corresponding certificates in `users.json`)
3. Switched Multiuser plugin on
4. Created new user, also regisered it with ZeroID
5. Logged in using older `master_seed` from `users.json`
6. ZeroID can't use my old single-user identity until it is really in `users.json` and Zeronet is started with `--multiuser_local` option

Just want to have an ability to convert my single-user local identity to multiuser one.

Not sure it was best place to post this, but I can't figure out where is better one, but it looks like ZeroNet itself recognizes old single-user identity correctly (I can be wrong though).

Thank you!