I agree with Bjorn, get them down to one deep and feed them a solid food. Syrup will put too much moisture in the hive and if they are weak, they will most likely get dysentery with syrup. Sounds like just pulling the bottom deep may be the easiest.
If you can't get the Fondant, you can make yourself a sugar board -> http://robo.bushkillfarms.com/beekeeping/emergency-feeding/
I assume you only have a few hives and are willing to go a little bit beyond the norm to help them make it through. If the population is large enough, just providing Fondant/sugar should be sufficient, assuming they are healthy and you cut their space down. If the population starts to dwindle, you could also help them by providing some supplemental heating with a 7watt night light.