XPS 8960, use XPS 8940 SSD card

Yes, you can add additional SSD card or SSD drive to your 8960.  Your system has two M.2 slots and three SATA cages/ports designated for storage.  The maximum capacity could be lower in the specifications but you can actually use any drives capacity if prefer.

https://www.dell.com/support/manuals/en-us/xps-8960-desktop/xps-8960-setup-and-specifications/storage?guid=guid-12cf2b7a-17bd-4537-b0aa-c1f3aee7967a&lang=en-us

Re: I would like to take the SSD card from my 8940 and put into my 8960 as an additional SSD card.

OP likely has a Dell m.2 ssd in 8960 as boot drive and wants to move the old m.2 ssd from 8940 to 8960.  In this scenario OP would have two bootable m.2 ssd.  OP may want to boot from the old m.2 to use old desktop icons and run old games. Before doing this make sure bios>sata operation is set to be AHCI and not RAID. I would remove the new m.2 ssd first then install the old m.2.  Make sure 8960 can boot from old ssd. It will need to adjust settings (drivers etc) to run on 8960 because its current settings are for 8940. When it boots, restart pc to make sure it can still boot a second time smoothly. Then confirm bios is AHCI. Power off and reinstall new ssd. Now pc may offer dual boot screen. OP needs to choose the old ssd as boot drive. If that works, OP can continue using pc in this way or choose to wipe new ssd to make it blank.