Defining your own command?

Is there a way that I can write a macro that will define my own command? For example,

Define command "/mycommand" as:
/assist Character1

And then write in another macro:
/mycommand
/cast Fireball

The reason why I would want this is I could write three versions of "/mycommand", with each version having a different character to assist. Then I can use whichever one is appropriate for which characer that I want to be the current leader.

This could probably be done with a custom addon, but I'm pretty sure you can't do that with macros. I'm not an expert on either though.