Or you mean the assertion
If error come out due to max execution time, it is necessary to adjust to a lower value the number of users to be added x time.
Refresh manually from browser: it will "reset the counter" of the transfer procedure.
It refer to the fact, that like in your case, if there are many users to be added, let say 500.000, you should may try to start adding
5000/6000 users (or even 10000 users, it depend on how much your server is powerful) and see if the transfer return error.
Let it run, maybe it will require minutes, until the browser run and you if you have no errors let it do the work (transfer).
Once you'll see after sometime, that there is the message to continue with the transfer, then continue adding same num of users per time.
If you get error on the first try, no problem: restart the process.
It will report users that have been already imported and will follow on add the not existent (and will run faster until no new users to be imported are found).
The point is to find out how many users per time your server can import.
And yes, an easy automation of the import could be coded. I will maybe do it easily as possible!