You can always manually add additional dimensions or metrics direct in PAS. It may well be the case that you have a range of metrics that are not sourced from BW and do not have the same dimensionality as those in BW. Technically no single metric can have more than 12 dimensions but in practice if you have more than 6 dimensions on a metric it will be very difficult to use simply due to of the complexity of the data ( a human issue not a technical one!).
In technical terms BW is 'just another data source' but because BW is so central to SAP environments it makes perfect sense for SAP to have prebuilt the interface into BW. Equally it is quite normal for Actual values to be sourced from a structured source such as BW while the Target cvalues are either loaded from a different source or entered via the manual data entry Entry & Approval component.
Regards
Colin
