importing config file into 11.4.x bigip.conf

Yes, this can be done in v11.4.x. I recommend not editing the bigip.conf file directly to avoid any mistakes that might make it unparseable. Instead, you can make a file some place like /var/tmp/ that contains just the elements that you want to add. Then within tmsh you can use a command like:

load sys config file /var/tmp/foo.conf merge

If that succeeds, the new config will be merged with the running configuration - and if it fails, it should fail as a whole and not change the running configuration. You can also supply the "verify" flag to have the system test the load without making any changes.