This is not so much a Bioperl question as an inheritance one.

You will need to write a constructor which takes a BSF::G and returns your object. It will look something like

sub new_from_generic {
    my $class = shift;
    my $generic = shift;
    my $self = bless($generic, $class); # to change its type
    ...
    return $self;
}

Note that the whole reblessing thing assumes that you are actually inheriting from BSF::G. While I'm not a huge fan of inheritance vs. containment, in this case, you're probably best off doing so, since so many of the BioPerl modules use the Bio::Seq::I interface.

As far getting them generic Bio::Tools::GFF to return your subclass, you'd also need to wrap or subclass it to convert the BSF::G output before handing them off. Honestly, I'd suggest just wrapping it unless you're making heavy use of the various parts of the GFF package, instead of just using it as a loader.