It itsafe to heat a mobile home with a wood furnace and can you tie into the excisting duck work?

Usually no it is not safe UNLESS the wood stove/burner is listed by UL or CSA for specific use in a mobile home. It also has to be installed exactly as the UL or CSA listing says and it cannot be connected to any other duct work unless it says in the instructions it can be done. Be very careful! Most wood burning devices are not listed for use in a mobile or modular home and if use in such can be a serious fire hazzard.