yes it is possible.
you need to write a script that will process 'on connect' event.
since, on connect the network name is in $network, you can
test that variable in script, and if its (base on you naming):
if ( $network == site1) {
do the /joins needed for this net here
}
if ($network == site2) {
do the /joins needed for this net here
}
notes:
see /help on connect and /help /join for more info.
for /join (from help /join}:
Some channels may also have keys ie. a password, which you need to specify when using the /join command.
example: /join #irchelp trout
If this is not enough information to get you started, let me know, will put up a pastebin example of how to do the code.