It would be a really good idea but yes, a 6 car 185 doesn't fit in South Parkway. At least headig in to Liverpool the doors on the back car heading in to Liverpool are locked. The guard comes around and warns people, or at least should do. Never got a 6 car set out of Lime Street to South Parkway but it must be the same.
It is still a very good station though apart from having only 3 superloo's for all the people who pass through. If you live in the area it is an excellent transport hub for entering or leaving Liverpool city centre. Could do with a much bigger car park but there isn't anywhere to put it.
Just as an Edit I don't think a 6 car 185 fits in Warrington Central either. I think small extensions have been built in case emergency evacuation is needed. Both stations are hemmed in by bridges and junctions.
Warrington central could easily be extended to the west.