Hi Anand,
The peripherals mapped to Port F (SPI0, PPI and Timers) don't use the Pin Muxing Add-in because Port F doesn't have a Function Enable register (FER). The peripherals on Port F are enabled directly from the peripheral control/config registers. See Table 14-2 in the ADSP-BF538 hardware reference manual. TWI has dedicated processor pins so there is no Pin Muxing required for TWI.
Regards,
Matt