No, you can’t. The pc comes with only 1 3.5” HDD bracket and it is used by the HDD that comes in the config you ordered.

@speedstep or @Tesla1856 or @r72019 might be able to let you know the part number to search online.

Silverstone adapter allows for 2 X   2.5 drive where a single 3.5 sits.

R494D from dell also does this.

https://www.amazon.com/SILVERSTONE-SDP08-3-5-2-5-Inch-Converter/dp/B002BH3Z8E/

https://www.amazon.com/Sparepart-Drive-Caddy-R494D-F767D/dp/B007Y8JI4U/

You squeeze the tabs with hooks after dropping in from the top to lock into place.

The drive is held in by prying the 4 other tabs apart to snap the drive in.

Although the R494D can also snap into there to hold 2 X  2.5 inch drives.

The R494D is INSIDE the 3.5 caddy in the video below.

To get it back out you have to remove data and power cable then squeeze in and slide back then lift out from the top.  Optical drive must come out before you can reach it.

https://www.youtube.com/watch?&v=6NBaAvoUt0A

DT CADDY

It supports 2 Drives whether SSD or not as long as they are 2.5 inch.

The catch is that you need sata power splitter and another sata cable.

DELL N701D cable.

https://www.amazon.com/Genuine-Dell-FOXCONN-SPLITTER-Desktop/dp/B00T74H5OE

  A Silverstone adapter has all the mount points of a standard 3.5" hard drive, and it's only $12 with free shipping. It holds two 2.5" devices stacked.

Hope this isn't duplicated. Received a post error.

Is there a 2.5" to 3.5" adapter?

I don't know why Dell would put 2x 2.5" drive bays in a desktop computer. I would rather have a single 3.5" bay. 

The R8 has 1 x 3.5" bay, and 2 x 2.5" bays 

I think what he wants is 2x 3.5". Well, this is a gaming desktop . Spinners aren't for gaming. Just look at PS5 announcement today. SSD FTW!

Similar situation here. I've bumped my R8 to become a storage/ Chrome workhorse and the custom rig gets all the games played on it. Now I have 2x 3.5" HDD but I can only plug in one at a time.

One of these days I'll whip out the mounting tape and velcro straps to paste the other 3.5" on the case floor.

I know what's in there, I have the .pdf and I have opened it up.

I don't need *another* SSD I need a storage drive.

AFTER

The 3.5" caddies for this chassis are like $10 on ebay.  Just swap it out with one of the 2.5" caddies since they are interchangeable.

Awesome, this is what I was looking for. I'll check ebay. Thank you.

Apologies if my question was not clear.

Alienware Dell Aurora R5 3.5" HDD Hard Drive Caddy Tray Bracket 13P1-4YN0G01

This fits perfectly on the left side to replace one of the 2.5 drives that comes with the R8. I just did it and it was seemless. 

You can get 2.5 inch hard drives.  Unless this is larger than 2TB its not an issue.

https://www.dell.com/support/manuals/us/en/04/alienware-aurora-r8-desktop/alienware-aurora-r8-service-manual/before-working-inside-your-computer?guid=guid-5d3b1051-9384-409a-8d5b-9b53bd496de8&lang=en-us

https://www.amazon.com/Seagate-BarraCuda-Internal-Drive-2-5-Inch/dp/B07D99S8Z7/

Seems that its on newegg

https://www.newegg.com/p/1B4-019U-00028

@r72019 

how did you take off the 2.5" HDD tray? I would like to remove both of them from the chassis to give the graphics a bit more breathing room. Service manual does not mention how to remove the HDD trays.