In case you haven't found it yet, there's a commandline tool (./dspace user) which can be used to pre-create DSpace user accounts with a default password. The docs for it can be found in our documentation at
https://wiki.lyrasis.org/display/DSDOC8x/Managing+User+AccountsIt's also possible for site Administrators to manage this via the user interface if your enable the "Impersonate EPerson" feature on the backend (again see the docs above).
However, if you wanted to do this in any "bulk" manner, it might be easier to just use the commandline tool.