Hi Asmaa,
1) The OQPSK modulator provides a baseband signal, not a passband signal. The motivation behind baseband processing is speed. It is much faster to generate or simulate a signal at baseband frequencies than at 2.4 GHz. Why exactly are you looking for passband operation? If you want to do the actual transmission at 2.4 GHz with a radio, then you can simply pass the baseband signal to the transmitter block (or System object) and this transmitter will do the upconversion to 2.4 GHz for you. No need to start with a passband OQPSK signal.
2) Be aware that ZigBee and IEEE 802.15.4 use half-sine pulses. The OQSPK tools of the Communication System Toolbox upsample the input signal (as of R2016b; see doc note), and a subsequent filtering operation will probably not produce the desired result. You will be better off if you conduct a joint modulation and filtering operation in this order (e.g., with a MATLAB Function block, or with 3 blocks in Simulink):
a. Split bits to odd (I) and even (Q)
b. Filter both I & Q streams for half-sine pulses
c. Delay the even-bit stream (Q) by half a simple and make a complex waveform I + j*Q
Tasos
