Missing network SuperAdmin - How to recover?

I've been assigned by a client the development/redesign of a multisite installation (previously maintained by a web agency).
I've been granted Administrator rights, but I do not have Network Administrator rights and the previous agency is of no help while trying to recover the user (it doesn't appear any SuperAdmin in the Users list).
I obviously have full access to installation files and to DB via phpMyAdmin.
Is there a way to assign network SuperAdmin rights to an existing user?