Afaik, you create an alternative using the --install option of the update-alternatives command. You can add this command to the package post-install scripts if you wanted to register a package as an alternative.
You can make your package the default on installation through the --set option or by putting the alternative group in automatic mode and ensuring your package has the highest priority.
There is no central repository, just the two two directories used by this command:
A directory, by default /etc/alternatives, containing the symlinks.
A directory, by default /var/lib/dpkg/alternatives, containing update-alternatives’ state infor‐
Relevant snippets of the update-alternatives man page:
--install genname symlink altern priority [--slave genname symlink altern]...
Add a group of alternatives to the system. genname is the generic name for the master link, sym‐
link is the name of its symlink in the alternatives directory, and altern is the alternative
being introduced for the master link. The arguments after --slave are the generic name, symlink
name in the alternatives directory and the alternative for a slave link. Zero or more --slave
options, each followed by three arguments, may be specified.
If the master symlink specified exists already in the alternatives system’s records, the informa‐
tion supplied will be added as a new set of alternatives for the group. Otherwise, a new group,
set to automatic mode, will be added with this information. If the group is in automatic mode,
and the newly added alternatives’ priority is higher than any other installed alternatives for
this group, the symlinks will be updated to point to the newly added alternatives.
--set name path
Set the program path as alternative for name. This is equivalent to --config but is non-interac‐
tive and thus scriptable.
Switch the master symlink link to automatic mode. In the process, this symlink and its slaves
are updated to point to the highest priority installed alternatives.