glibc supports `[SUCCESS=merge]` since version 2.24.

 From nsswitch.conf(5):
The `merge` action is used by services like `nss-systemd` to attach
extra entries to those returned by the standard services. From
nss-systemd(8):

```
group:   files [SUCCESS=merge] systemd
```

Such actions are best seen as "prefixes" of the non-standard services
that add extra entries rather than as "suffixes" of the standard
services that provide the base entries. Thus the .nss file of the
non-stardard services should have a way to specify that this action
should be placed before the service that is being installed.