Yeah, you might think something like SVG to PIC might be an option but the very way they work is too different. Particularly in how filled shapes work: there are no "shapes" in an SCI picture, and there are no flood fills in off-the-shelf vector systems, only fills for those shapes.
So if you have to limit yourself to only drawing the shapes and doing the fills "in post" (that is, in SCI Companion), even then you'd have the added limitation of only being allowed to use simple, straight lines. Sure, you could have entire paths from A to B to C, instead of going A to B then starting a new line from B to C, but I feel like you'd still lose basically every advantage to using an external vector editor.