Will adding an external MOSFET on the switch pin of a boost converter IC increase the output capacity in terms of current?
It will work ok to a point, but the P channel MOSFET is a high-side switch; yours is on the low side.  It will turn on only until the source pulls down to the level of the gate, plus (less negative) minimum turn-on voltage.  This will cause a high Rds(ON) resulting in higher power dissipation of the FET than you would expect.  You need to connect the output to a PNP to pull up the gate of an N channel switch.