Can I remove the whip antenna of an Xbee Pro S2B and attach an external antenna?
The Patch antenna is unlikely to have higher gain than a monopole and although the rubber duck is about 2 dB more gain than a monopole (whip) the cable losses might make this venture pointless. 

2 dB more antenna gain will give you theoretically about 26% more range i.e. it might extend the range from (say) 100 metres to 126 metres and this doesn't appear to tally with what your want: -

My project needs a lot more range


You can easily lose 2 dB in a few inches of coax at 2.45 GHz so bear this in mind.