Sorry ... I misread your post ... you want to add a drive that is NOT part of the RAID 5.  I guess I got confused by the question of whether it would slow the RAID 5 to the new drive speed, to which the answer would be no.  You can add a disk without making it part of the RAID 5, but if you are using a PERC H700, then you can add a drive ... although it does not have to be part of your existing RAID 5 or in a RAID 5 of its own, because the PERC doesn't support non-RAID, it MUST be configured in SOME RAID array ... by itself, it would be a single-disk RAID 0.  It will operate at the speed it is capable of, and the RAID 5 will operate at the speed it is capable of ... if you are copying from the RAID 5 to the single-disk RAID 0, then the RAID 5 write speed will be limited to the transfer speed and write speed of the single disk (obviously, an SSD would be faster).
If I have a Raid 5 array in my R710 and want to add an additional drive that is not part of that raid array, will that force the Raid 5 array to operate at the speed of the new drive -- aka the lowest common denominator.
Adding New Storage -- Disk Performance Question