Currently we are using FAS 3250 with four 2TB Hard drive Shelves Model DS4246-0724, I want to expend this with more four 1 TB Hard drive shelves model DS4246-0724. Please tell me is it possible that we can expand it in this scenario.
While it will work, it's always recommended best practice to never mix drive types, sizes and speeds within a single aggregate. Furthermore, if for some reason any of the 2 TB spares had to be used in place of the 1 TB drives, it would downsize them.
Do you have a required need to expand the existing 2.0 TB-based aggregate or can you afford the process of creating a new 1.0 TB-based aggregate (and move data around if you need to)?
Also - keep in mind that the max aggr size for the FAS3250 is 240 TB, so if you add those four 1.0 TB shelves to it, you'll be getting pretty close to the max.
The recommendation that DOMINIC.WYSS made for creating the new aggregate and its raid group size is spot on.