Is there any block that moves other blocks around it when being pushed by a piston, like a slime block in the latest version of vanilla Minecraft in Hexxit?
There are mods with the "slime block" function, the first one was Redpower with its Frames. However as RedPower was not updated, there was a lot of small mods to update some parts of it.
You should look for some Frame mod, Redstone in motion or similar, there is a lot of them around, I was not using any, so I cannot tell which suits the best.
However - You will need to alternate you modpack by adding that mod, so be sure both server and clients will have it properly installed.