is it possible to change mac layer of xbee pro modules?

which XBee module are you referring to?

(PN:XBP24CZ7RISB003) &
XBee-PRO ZigBee Through-Hole
I want to implement beacon enabled mode of 802.15.4. Is this possble?

No, that is not an 802.15.4 product. That is a Zigbee Enabled product which has 2nd processor on it. It is referred to as a Programmable XBee. Ember does not offer an 802.15.4 Stack to work from.