No, not really. RGSS3 has a totally different mapping setup and event setup. First of all, different event commands and a different interpreter. The interpreter scripts in both engines are what make events run the way they do and the difference between the two's interpreters are way different. Second, VX Ace has a different mapping style. Layers work completely differently. XP has three layers while VX Ace works with layers depending on the tilesets that are chosen. And the underlying engine's tilemap class handles and draws this data differently than it's counterpart. My understanding of RPG JS is that it's based around XP so we could try changing the converter script to convert the data, that's not really the difficult part, it's just that the engine wouldn't know how to handle this data, so it'd have to be rewritten so it would handle properly.
A few other differences is that Items, Weapons, Armors, and Skills are all child classes of a class known as BaseItem. Not to mention the difference between the engine's default scripts.