One account cannot hold distinct chunks of locked veBAL with different expiration dates.
If you lock additional BPT to increase an existing veBAL balance, you must also choose a new expiration date for your total veBAL balance. The earliest option is the original expiration date.
To acquire veBAL, you'll need to do the following:
Provide liquidity to the the 80/20 BAL/WETH pool on Ethereum
In doing so, you'll need to choose your lock period. The longer you lock, the more veBAL voting power/weighting you get, but the longer you must wait to unlock your BPT
To extend your lock time, go to the veBAL page and follow the same process as locking your 80/20 BAL/WETH BPT; this time, however, you do not need provide additional 80/20 BAL/WETH BPT; instead, enter a later date in the calendar selector to extend your lock period and acquire more veBAL.