I tried that same thing when I put my tree sponge in but a lot of the bag water still got into my tank...
Nothing bad happened though.
Anyway,
I read a good idea that you could try if you don't want any bag water in your tank...
You might want to acclimate the sponge in a seperate bucket...
Just put the bag with the sponge in the bucket (don't worry about pouring the water out into the bucket or anything... it can stay in the bag) and every so often add some tank water to the bag. After adding a few cups empty out about three or four cups of water.
Continue to do this for a while refilling and emptying the bag over and over and eventually most of the water in the bag will be from your own tank... Then just dip the entire bag into the tank and once submerged, let the sponge out.