BIG-IP send all syslog messages as local5 facility

i dont believe this is easily possible. the BIG-IP uses many different facility values itself by default, so changing those will confuse system that asume the default ones.

you might be able to overwrite something by tinkering in the syslog-ng.conf but i wouldnt advise that.

i assume you want this to recognize the logging better on the syslog server? isn't that possible on source IP or such?