Yes, you can add 2 more 3.5" HDDs inside the chassis. When install 2x 3.5" HDD, the right-angle cables are required to avoid interfere with top cover.
Here are optional right angle SATA power extension:
CBL-0143L ( 1x4pin to 1 x SATA S-RA SATA power extension )
CBL-0082L ( 1x4pin to 2 x SATA S-RA SATA power extension )
Option right angle SATA cable: CBL-0231L 70cm round S-RA SATA cable.
Please contact Tech Supports if you want to know which cable is best selection for your chassis.