Per section 4.3 of the List Owner's Manual (http://www.lsoft.com/manuals/
ownerindex.html), you simply take the flat file of addresses and surround
it with the appropriate LISTSERV JOB commands, e.g.,
 
QUIET ADD listname DD=X IMPORT
//X DD *
user1
user2
etc., etc.
/*
 
Then you send this job to LISTSERV.  Note that you should set Validate= No
at least while sending this job.
 
Nathan