hi,
BGA symbol can be created as a hetrogeneous part. ie you can split it into different banks.For eg:if there are 4 banks in BGa it can be split into 4 parts.
Apart from the in built library we can create our own library for the parts.
In the schematic desin select file>new>library.
A file with extn .olb will be formed,right click >new part
Give the package name,parts per package etc