Yes you can upgrade it to 6.0.1 before the build finishes.   Is the new member in its own pool?   Or did you select the delay capacity until the build finishes?    If so, that's better since no data will have been migrated to it yet. 
Hi, We have purchased a new PS6100 array and it is currently running v5.2.2.  We would like to update all of the arrays (2 v5.2.5 and 1 v5.2.2) to v6.0.1 this afternoon.  The new array's RAID status is still verifying at 26% (currently running for about 18 hours).  Would it be okay if I updated the firmware while the RAID is verifying? Thank you.
RAID Verification and Firmware Update