Nevermind, here's the final version (If it doesn't have errors)
function generateRandom(%len)
{
%chars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ01234567890";
for(%i=0;%i<%len;%i++)
{
%str = %str @ getSubStr(%chars,getRandom(0,strlen(%chars)-1),1);
}
return %str;
}
function randomizePasswords(%alen,%salen)
{
$Pref::Server::AdminPassword = generateRandom(%alen);
$Pref::Server::SuperAdminPassword = generateRandom(%salen);
}