Can you add more vitamin C to make it a 60% solution? What would happen?

If 20% is good, more must be better, right? Not necessarily. A medical university that did studies on the percentages of vitamin c applied topically concluded that 15% is the minimum to see results and 20% percent maximum for best results. Anything under 15 is about useless and above 20 won't absorb into the dermis. I've used 30% before but got no better result than a 20%. I don't think 60% would be beneficial and may strip your skin or even damage and or discolor it.

Can you add this to a non water based formula? Such as beeswax and oil based creams?

Mix it with your water or try these alternatives rosewater, aloe vera juice and then mix it in to your cream lotion etc.

What is the expiration and what is the safest place to keep the unused portion?

As long as you seal it with no or little air in the bag you should run out way before it ever expires. I've been on the same bag for about 6 months or so and its as potent as the day I bought it. I keep it in the bag it came in along with my other diy products in a paperbag under the bathroom sink cabinet.