Hi! I'm new to Craftster (and a little new to sewing) and hoped that come more experienced crafters can give me some advice. I'm pretty good at sewing from patterns and have made myself a few skirts, blouses and dresses but now I have a new problem that I can't figure out. I ordered a playsuit from Asos, and it's lovely but the shorts are just too short for me. They're okay for winter/autumn when I can wear thick tights but for wearing them in warm weather I'd like to add some material to the bottom.

My problem is that this is not something I've ever done before and I don't really know where to start. Here is a link to the playsuit: http://www.asos.com/ASOS/ASOS-Pussybow-Playsuit-in-Bird-Print/Prod/pgeproduct.aspx?iid=2477608&affid=3504

I'd like to add about 12cm (5 inches) if I can but I don't know if that's really possible. The playsuit is 100% viscose and the shorts are fairly wide.

What kind of fabric should I use?
Can I machine sew it or do I need to do it by hand?
Aesthetically, I thought I'd just try to match the black of the fabric, does that seem like the best choice or would I be better off trying to complement the black/grey?
Any suggestions for measuring fabric, common mistakes etc.?

Any advice would be great, even if it's just "Sorry, not likely to work". Thanks!