If you are expanding the existing arrays, you will need to use 15k SAS drives. Mixing different speeds of drives in a single array will lead to strange results - at best, everything will slow down to the speed of the slowest drive. NLSAS drives are basically 7.2k RPM SATA drives with a SAS controller on the drive, so they are much slower.
If you are simply adding another array, use whichever drives work for your price, performance, capacity and reliability requirements.
I dont have the exact procedure for expanding the arrays in front of me, but its basically insert the drives, open the Dell OpenManage Server Administrator (OMSA) software, choose the existing array and follow the directions to expand it. Should be well documented in the openmanage help files.