Perhaps you mean macros?  The method changes from assembler to assembler, so you'll have to tell us which one you're using.
Macros are a special sort of command that changes text.  For example, you could make a macro where "thisisamacro" would be equivalent to "ret".  Certain types of macros have other features, such as parameters.  One such example is bcall.

It sounds like you're talking about modifying what the interpreter itself does. Modifying the interpreter can involve some heavy OS modification, which is pretty much the last thing you want to be doing if you're new to ASM. However, since I know you'll be curious, here's kind of a general overview of what would be necessary:

First, to parse a token, the interpreter steps through an array containing all of the tokens and does some fancy parsing in order to transform those commands into something it can understand. For example, something like 3x+3 is transformed into another format that's easier for the computer to parse. For simplicity (and because I have no idea how TI calcs parse expressions  ), we'll skip that part and just go to single token parsing like Dim(L₁). In order to parse this, the OS loads the arguments into the proper memory locations. It then looks up the Dim( token in a big table and sees that it's supposed to perform an operation on the arguments and finds the addresses of the proper system call(s). It then calls those functions and when they're done, waits for the next token to come along.

If you can't tell, in order to change the tokens, you'd either have to do a parser hook which intercepts the token and parses it by itself or modify the OS itself. The first is possible, but still difficult. The latter is well, difficult to judge. Let me just put it this way: In general, the most complex and massive program a programmer can write is an OS. Modifying an OS isn't easy to do properly without some serious knowledge of the OS in question. Also, it should be apparent that modifying a system call may have impacts beyond what was intended if the call is used elsewhere.

Well, you'll notice that I did mention those

However, OS patches don't permanently reassign the OS commands. The parser hooks are uninstalled after every RAM clear.

It is possible, just take Omnicalc, for example. Omnicalc changes the parser by using the parser hook.
http://wikiti.brandonw.net/index.php?title=83Plus:Hooks:9BAC
^^ is about the parser hook

Ah, okay. Well I think I'd like to learn how to do that once I understand all of this a little more.

Thanks for all your help, everyone.

Using a parser hook is the same as asking for a headache without any aspirin.  Many of us are more than happy to help you out, but I recommend staying away from the parser hook until you become more fluent in ASM.