Do you mean fit a ITX board in a 5.25" bay? If so you can't. The 'CD Drive' is just shy of 6" (15cm) but an ITX case needs 17cm and it is square. Yo'd go quite far back into a ATX case even if yoy mounted the ITX vertically.
Or have I misunderstood?
I just fitted my CD bays to take extra 2.5" SSDs, not that I've actually populated them yet so really they are storage drawers!
ETA
Just thought, you could add something smaller than ITX, but that might mean it is a NUC or an AIM based system. NUC you can Google, they are Intel barebones systems, and the AIM stuff is things like the Raspberry Pi- known as Small Board Computers. Some of the SBC do use Intel x84 chips but I think they are all out of stock at the moment, pretty sure the Latte Panda, which cost about £130 but was quite well featured is out of stock everywhere.
If what you are talking about is something like an external drive bay though, then either of these choices would fit with space to spare.
The Raspberry Pi3+ is widely available and costs £35/$35 but is also quite limited. USB2 only, where as the Latte Panda has/had SATA and M.2. Also the Pi is AIM based- it can make an OK Kodi box and a limited Linux based desktop.
