Install the xattr utility tool:

apt install python-xattr

Assuming the root of the mergerfs filesystem is /storage, use xattrs to view the pseudo .mergerfs file:

cd /storage
xattr -l .mergerfs

returns:

user.mergerfs.srcmounts: /mnt/data/disk01:/mnt/data/disk02:/mnt/data/disk03
user.mergerfs.minfreespace: 21474836480
user.mergerfs.moveonenospc: true
user.mergerfs.policies: all,eplfs,eplus,epmfs,erofs,ff,lfs,lus,mfs,newest,rand
user.mergerfs.version: 2.13.1
user.mergerfs.pid: 91089
user.mergerfs.category.action: all
user.mergerfs.category.create: eplfs
user.mergerfs.category.search: ff
user.mergerfs.func.access: ff
user.mergerfs.func.chmod: all
user.mergerfs.func.chown: all
user.mergerfs.func.create: eplfs
user.mergerfs.func.getattr: ff
user.mergerfs.func.getxattr: ff
user.mergerfs.func.link: all
user.mergerfs.func.listxattr: ff
user.mergerfs.func.mkdir: eplfs
user.mergerfs.func.mknod: eplfs
user.mergerfs.func.open: ff
user.mergerfs.func.readlink: ff
user.mergerfs.func.removexattr: all
user.mergerfs.func.rename: all
user.mergerfs.func.rmdir: all
user.mergerfs.func.setxattr: all
user.mergerfs.func.symlink: eplfs
user.mergerfs.func.truncate: all
user.mergerfs.func.unlink: all
user.mergerfs.func.utimens: all

From within the root of the mergerfs filesystem (eg. /storage)

xattr -w user.mergerfs.srcmounts '+>/mnt/data/disk04' .mergerfs
 
or, for adding two disks use:
 
xattr -w user.mergerfs.srcmounts '+>/mnt/data/disk04:/mnt/data/disk05' .mergerfs