The A: drive is just a drive letter as such, but the hardware side of it is different. If you already are full on the hard drive slot side, (usually 4), you can't connect a hard drive to the floppy port, they are not the same.
If you don't have 4 HDD's, then you can add more, and use any letter you like to allocate them, or change the existing letters to something new.
You may run into problems if you try to allocate A: to a HDD, there are some applications that are hard coded to see a floppy on A:, and the results of trying to access a HDD on A: are at best unpredicatble, and at worst downright dangerous, especially if the application has it's own inbuilt driver for the floppy drive.
If it's only the physical space that's the issue, that should be OK, but you may need to check that the power supply has the right connectors for the HDD, the floppy drive connector and the HDD connector are usually different. I've seen converters to go from HDD to FLoppy connector, but I can't recall seeing the opposite way round.
Hope that helps