Hi,
There is no support in HW to extend break delimiter.
And the SDK/RTD drivers do not do it either.
Similar has been discussed with the SDK team before.
This was their answer:

So adding a delay loop as above is the only option.
This way you can't adjust the exact time of Break delimiter length. It is necessary to set the number of loops considering CPU Clock and UART Baudrate, and deviation may occur depending on CPU conditions.
BR, Petr