In Excel how can I add data to an existing cell?

I have a row of cells that have a truncated zip code. For some reason this system truncates preceeding zeroes. I need a formula that will allow me to add a zero to the front of a 4 character cell. An example in J2 I have a 7075 which should be 07075. I tried looking at the Right-Mid-Left commands but I can only disply the contents of the cell I can’t get the zero to insert. Any help would be greatly appreciated.