I am pretty sure FrontDesign uses a font format typically known to be used in CAD programs.
My understanding is that they use these types of fonts because the engraver cannot follow standard Windows/Mac fonts (TrueType/OpenType fonts).
Long story short, yes, you can use other fonts. At the end of the day the engraver will follow whatever 'path' you draw.
If you open up Adobe Illustrator for example and create a path with the pen tool (e.g. a squiggly line or the outline of a circle), and then subsequently convert that line path to a HPGL format and import it into FrontDesign. The engraver tool will follow the path of the line drawn and the width of the engraving tool will be reflected as an additional width to the line. This means that the line that you have drawn, although it may appear to be 1mm thick in Illustrator - If you choose the Cutter 6.0mm engraving tool for example, it will create a 6.0mm 'version' of the 1mm line you have drawn.
I guess you can also start to see how external fonts can be created. If you choose to use an external font, you will have to convert it to an outline (e.g. the font has no fill and an outline of the font can only be seen). In Illustrator this is done by going to "Type -> Create Outlines" I believe.
To replicate the fill, the outline of the font is duplicated inward until the font is completely filled with lines (this means the engraver will follow each line path until it reaches the center).
Hopefully I didn't confuse anyone,
Braeden