RPG system not supported

How can one go about having an RPG system included? I'm trying to do a battletech RPG so I can have some visual aids for my players, and maybe so we can play from a distance if need be. Problem is it looks like I'm having to build the system in fro the ground up, that seems a bit difficult, I just need some general help.

Roll20 is a system-agnostic tool.  It's intentionally designed to not be specific to any particular game. When you say "Build the system from  the ground up", what are you referencing exactly?  All Roll20 GM's have to setup their own graphics, player journals, tokens, etc.  The only exception to this is the pre-made adventures available for purchase from the MarketPlace.

Ok. That's kinda what I needed to know. "With building the system" I'm basically referring to macros and enemies. Some of the Macro examples seem to incorporate elements from a system, like Armor Class, so with Battletech using a different system, can I build a macro for a specific weapon or should I just make it for a generic roll, against a target number? Also can I build a Macro to compare a player's weapon damage against a mook's damage?

You can build just about any macro you want, permitted that you know how to code.