DSPFD FILE(QCLSRC) TYPE(*MBRLIST)
Or, more elegantly, use this API: QUSLMBR.
The simple answer above is correct. But why would you want to store the list in a database file? The connection to a “subfile” doesn’t provide any useful information since you can build a subfile from a *MBRLIST member list ‘on the fly’ or from the API whenever it’s needed.
I also generate files from member lists, so I realize there are reasons. I’m asking mostly to get you thinking about potential alternatives. Building subfiles isn’t sufficient reason (IMO) to add new database files.