If the echoes have explicit restrictions in their rules, e.g. "this
echo should only be available for coordinators and hubs", then you
are probably out of luck, unless you can guarantee that the newsgroups
are not distributed uncontrolled.
If the echoes in question are "normal" ones, i.e. you have right to
give them to your BBS users, then you can talk this way: hey, you
see all my subscribers as the users on my Fidonet BBS. You
should not be bothered about the technology I use. For you, moderator,
I am a normal fidonet node running a BBS with numerous users.
Of course, if you cannot prevent spam from getting into fidonet echoes,
people will be annoyed and kick you off sooner or later...
Eugene