the 540mt has 2 internal 3.5" drive bays so adding in another drive is easy.
Any 3.5" sata drive will work and they come in various sizes up to 4TB thought to me the 3 and 4TB drives are too new to trust yet. I have several 2tb sized drives. The early Western digitals (drive model# ends in ears) have all failed (all 3 of them)and been replaced and the new models have worked flawlessly for well over a year now. The other 2tb drives I have are samsung F4 and I have not have had any issue with any of the 5 of them at all and they a few of them are over 3 yrs old.
if you do decide to use a 3 or 4 tb drive, please contact Dell to make sure your system will work fine with these drives or if there is special software you need to run first.