Is it possible to send syslogs and security logs to a hostname instead of an IP?

Yes, when you configure the syslog parameters it gives you the option:

[edit]

root# set system syslog host ?

Possible completions:

 <log-host-name>   Host to be notified

 juniper.net     Host to be notified

 other-routing-engine Send to log file on other Routing Engine

Where hostname is the fully qualified hostname or IP address of the server that will receive the logs.

Same with security logs:

[edit]

root# set security log stream TEST host ?

Possible completions:

 <ipaddr>       IP address/Host name

+ apply-groups     Groups from which to inherit configuration data

+ apply-groups-except Don't inherit configuration data from these groups

 port         Host port number

 routing-instance   Routing-instance name