Hello and welcome,
Do you intend to use it as transportation or just fun.
I had the opposite situation as you. I had an old kids mountain bike with only a front brake. This bike got the motor until I could find a bike that had better brakes and looked better.
I ended up buying a Kuluna Moon Dog cruiser, its got good brakes and is a 7 speed. It's also fairly comfortable to ride.
Yes it is possible to add a front brake, the easiest to add would be the caliper type also known as u-brakes. Your Schwinn probably has this type. For more info on brake see
this thread.
Don't forget about gearing for the peddling, it is more difficult to take off on the one speed bike.
As for the cables on the schwinn, you can place the bracket between the frame and cables so it won't bind your brake or shifters.