We can put a 3rd party apps new global PATH in /etc/profile, appending to the original $PATH, ok.
But.. I can see that the /etc/profile file is provided by a package: aaa_base. What would happen if someone upgrade the aaa_base?
The Q: How can we ensure that the $PATH stays somewhere, where an upgrade wouldn't modify it?
Since you have an existing /etc/profile.d directory (and presumably the corresponding /etc/profile or /etc/${SHELL}rc files that source files in that directory), I'd recommend placing an /etc/profile.d/3rd-party-app.sh and/or /etc/profile.d/3rd-party-app.csh file with the required code. If you are the packager of the 3rd-party app, you could include those files in the packaging so that they are installed, updated, and removed by the package manager. Otherwise, as a user of the software, placing those files there will make them unmanaged, and so unaffected by OS package updates.
UPDATE from OP:
https://www.suse.com/documentation/sles11/book_sle_admin/data/sec_adm_whatistheshell.html
/etc/profile Do not modify this file, otherwise your modifications can be destroyed during your next update!
/etc/profile.local Use this file if you extend /etc/profile
/etc/profile.d/ Contains system-wide configuration files for specific programs