No you can't. The S7 and S5 PLC's use different hardware and software. You'll need a new rack and a new S7-400 PLC to start with. You should be able to use the existing PROFIBUS I/O as it is. The software has to be rewritten. You can convert certain parts of the software with a tool from Siemens but this doesn't work for every function. It all depends on how complex your software is.
Do you have any other cards on your S5 rack or only an IM308?