AD Groups and sub group will only reflect in BO if you add the root AD group and its sub group both in BO.
I did not get your second requirement.
When u say "existing groups in BO which need to map too"
Are these Enterprise Group?
IF its Enterprise Gruop and you creating a AD group by same name and hopeing it to sink with the AD group that will not happen.
User get sync with there 2 Aliase.
Group name dont sync they are either created at BO level or imported from AD.