Transition it outside, to a sheltered spot. Once it is transitioned it can stay outside even in the snow. The only thing you really have to worry about is the pot. It might crack if it freezes. Most people keep it outside and either bury the pot in the ground or cover it with a thick layer of mulch. Most of the time if the pot cracks, it was because it was time to be replanted anyway. If the soil is loose and not a tight mass of roots, it has room to expand. What you can do since the plant is not that healthy is to repot it into a nursery pot and keep it outside to get healthy again before trimming the roots to fit in the bonsai pot again. Only use a pot or container that is large enough to fit the plant in, but not overly large or deep. You can trim the roots of the juniper if you need to get it to fit. You will also have a chance to check if the roots are rotting at the same time. If you have a local bonsai society, join the club. Usually members bring their plants to work on and the sensei will advise on training, potting, and care.
Junipers are like orchids, they usually die very slowly. In the orchid world we say orchids don't die, we kill them with love.
Happy gardening in Hawaii. Gardens are where people grow.