Paralleling MOSFETs or MOSFET and IGBT to increase capacity in QRSMPS
I haven't searched BUT it is quite likely that these parts are functionally equivalent and possibly pin for pin to ones from a range of other manufacturers. This is the sort of thing that eg TopSwith do - and various others. 

The lack of access to the gate makes paralleling hard. You could look at current in the Drain lead as a trigger signal for a parallel FET.

The functionally easiest method is probably to use the on-chip FET as a driver only for an external MOSFET. In order to use an N Channel lowside device you'll need a gate driver to invert the signal. Conceptually a high side PNP or small P-Channel FET with emitter to a say 12V supply, base driven by IC-Drain and new MOSFET gate driven from PNP collector. You then need a 12V supply for gate drive and some glue parts. New FET gate drive is slow and will probably need a jelly bean 2 transistor driver. Parts count starts to add up.  Not vast but also not zero.