Can a ppa repository be added to /etc/apt/source.list?

Yes, PPA's can be added to /etc/apt/source.list, in similar way to debian (deb) repositories.

deb http://ppa.launchpad.net/<ppa-name>/ppa/ubuntu <release-code-name> main 
deb-src http://ppa.launchpad.net/<ppa-name>/ppa/ubuntu <release-code-name> main

An example for WineHq:

Thought just matter of keeping things somehow easily managed, leaving /etc/apt/source.list only for official release repositories. All additional repositories goes to /etc/apt/source.list.d/. Easy to:

1. add (creating file then editing existing one, a partial solution to avoid duplicates too)
2. remove (with parsing a /etc/apt/source.list looking to related line)
3. backup/restore (using compressed archive of /etc/apt/source.list.d/ folder)
4. avoid breaking things with much edits targeting /etc/apt/source.list

PPA are always added to /etc/apt/source.list.d/ folder if wrote it in ppa:<user>/<ppa-name> form.

Reference: man add-apt-repository

REPOSITORY STRING
       REPOSITORY can  be  either  a  line  that  can  be  added  directly  to
       sources.list(5),  in the form ppa:<user>/<ppa-name> for adding Personal
       Package Archives, or a distribution component to enable.

       In  the   first   form,   REPOSITORY   will   just   be   appended   to
       /etc/apt/sources.list.

       In  the second form, ppa:<user>/<ppa-name> will be expanded to the full
       deb  line  of  the  PPA  and   added   into   a   new   file   in   the
       /etc/apt/sources.list.d/  directory.   The  GPG public key of the newly
       added PPA will also be downloaded and added to apt's keyring.

       In the third form, the given distribution component will be enabled for
       all sources.

Well, it seems only PPA's as shortcut goes to /etc/apt/sources.list.d/. add-apt-repository or apt-add-repository is a Ubuntu specific tool. All I can think of is just Ubuntu decision to keep personal PPA's out.

However you can modify it to use only /etc/apt/sources.list. It's a python3 script. Modify /usr/bin/add-apt-repository line:

shortcut = shortcut_handler(line)

replace it with this one below to resolve ppa shortcut form to a deb line form:

shortcut = shortcut_handler(shortcut_handler(line).expand(sp.distro.codename)[0])