DIY POE 8 port switch
Don't do it!  Here's some reasons why you shouldn't:


The isolation transformer is not likely to be able to handle the
current that PoE will push through it. 
You can't support the full PoE
discovery protocol without modifying the firmware of the Ethernet
switch. I am referring to LLDP.
Not supporting the PoE discovery protocol could allow you to
power a device that isn't able to handle it-- causing it to damage
the device. 
Careful current regulation is required to prevent
creating a fire hazzard.


I have to admit that I am making a couple of assumptions about your ability to design this thing properly-- assumptions that might be wrong.  I am assuming that you don't know how to design it properly because you don't apparently know how the Ethernet signaling, isolation transformers, and PoE discovery works.  

If you knew how PoE and Ethernet actually works then this project might be barely possible-- but hardly cost effective since there are other PoE switches and injectors that are not super expensive.  Even as a learning experience this has limited value, since there are better ways to learn PoE.