It shall be possible by creating a templates-custom for /etc/nethserver/eorepo.conf. One option could be something like this:
mkdir -p /etc/e-smith/templates-custom/etc/nethserver/eorepo.conf/
vi /etc/e-smith/templates-custom/etc/nethserver/eorepo.conf/99custom
# on this file, set repos' name (one per line).
signal-event nethserver-subscription-save
Check nethserver-subscription — NethServer 7 documentation
EDIT: On nethserver-dolibarr wiki you can find another example:
drop a file /etc/e-smith/templates/etc/nethserver/eorepo.conf/10dolibarr
{ # enable the stephdl-dolibarr repo push @repos, 'stephdl-dolibarr'; ''; }do a signal
signal-event software-repos-save