You'd need to modify /etc/samba/smb.conf and add the folder you want to share, with a similar set of configuration options as the other RetroPie share
[...]
[pi-home]
comment = pi-home
path = "/home/pi/"
writeable = yes
guest ok = yes
create mask = 0644
directory mask = 0755
The easiest way would be to enable SSH and use WinSCP to see the files on the RetroPie system, you'll have access to the entire filesystem.