That's exactly what I have been doing. I sometimes also incorporate doing one of the next week's longer jogs. That way I feel confident going into the next week knowing that I have run the new/longer time at least once.
Started C25K February 2013
Graduated C25K June 16 2013
i'd look for training plans online that meet your goals and work on a 5 day schedule. there are a ton of training plans online, so it's really hard to say what to do / redo without knowing your goals.
I would slowly extend the current the current 3 runs to get more weekly mileage (from say, 3 miles each to 3,3,4, then 3,4,4, then 4,4,4). After a week or two, cut back to 3 miles each and add a 4th day of 3 or maybe 4 miles. Repeat the process to get to the number of days and miles desired.
If your question is "can I do five days a week instead of three when following the C25K program?" then the answer really is no, because then you are no longer following the program. You can do it - but the program is pretty clear about the hazards of pushing harder than is recommended.
If you're in condition such that you can push the limits of the program by doing 80% more than is called for, perhaps it's not really the right program for you.
You can run 7 days a week. Just listen to your body if something is hurting. Mix up the surfaces you run on and the shoes you run in though.