Hi Manuel,
Unfortunately SSG550 does not support this feauture. As stated in C&E "High Availability":
Some system platforms, such as the Integrated Security Gateway (ISG) systems and
the NetScreen-5000 systems, allow you to combine the throughput of one or more
pairs of physical ports into a single virtual port. This virtual port is known as an
aggregate interface.
Only Secure Port Modules (SPMs) support this feature, and you
can only aggregate side-by-side ports that reside on the same module.