Enable this program to be used as an add-on/plug-in to other fully featured image editing programs both commercial and open source/freeware.

What would be the point of doing that. iResizer is standalone now and available to everybody.
As a pugin it would only be dependant to other software and for a smaller market.

Mostly plugins use coding from the program where it serves as a plugin. Developers make it an optional feature to a greater product. There is no sound reason for iResizer to do such a thing.