There are a number of external spell checker applications available, which function with programs like NotePad and Word
These will not, though, work with PFE.
The reason is that the spell check programs assume that the program they're integrating with use a Windows facility known as an Edit Control to display the text you see as you edit it.
In the case of NotePad, the assumption is completely correct; other applications, even if they do not use a standard Edit Control, may simulate its interfaces for the benefit of other programs.
PFE neither uses standard Edit Controls, nor simulates their interfaces, so external spell-check programs have no way to integrate with it. A future release may add enough interface simulation to allow this, though.