What kind of winter hiking you will be doing? Winter hiking that gets below 20 degrees? And how much time do you have to complete this. What is the condition of you current bag? How much suffering are you capable of? I'll assume that it will get cold and you don't have a lot of time. Buy a budget bag. Case in point. I made a 15 degree bag for my wife and we headed out for a winter hike. I made it 4 inches too short. Her feet cramped up the foot box compressing the insulation at her feet and head. She froze and the trip ended the next morning. Winter hiking is serious stuff and a miserable night will make you regret not buying a good bag.
As far as you question. Making an over bag out of nylon and insulation (primaloft/what ever else is out there) would be the best option.
Possibly another option is to go thrift store shopping for a down blanket (or a couple of them). I've found two of them that I'm going to steal the down for a summer bag. Use the blanket over your sleeping bag (or sew it into your fleece bag with nylon cover), wear all your clothes and make youself a better bag during the summer with the blanket when you are done with it.
Of course all these options are not as good as one dedicated winter bag. They will all be bigger and heavier and possibly not as warm. For all this effort, 100-150 dollars seems cheap (and I am a seriously dedicated DIY'er).
Or I could be completely wrong....
Scott