Unfortunately you can't combine them into one. the
instruction in Z80 Assembly. means return, it is not the same as in ti-basic! what it does is it very basically said stop the hex-program and let the operating system take over control (aka your program continues).
Z80 assembly and ti-basic are entirely different. You might wonder what Z80 assembly is in the first place so let me explain. The Z80 is the processor that is in the Ti 83/84+ (SE). You can look it up on wikipedia. A Assembly language is a, what we call, low-level language. Low level languages don't have If's, Then's , Else's, Disp's etc. It strictly consists of moving data around and doing something with it. Once again I recommend reading up wikipedia about assembly languages.
There is no hex equivalent to IF. The only conditions hex code can react on is
Zero //the previous executed statement resulted in zero
Non-Zero //he previous executed statement resulted in a nonzero value
Carry // the previous executed statement caused a carry bit
Non-Carry //idem dito but inversed
Parity // I don't fully understand this one myself
Overflow //in order to understand this you must know about registers and binary, too much for now.
As said you can not combine ti-basic and Z80 asm. You can try but I am quite sure it won't work.
You must realize that ASM (short for assembly) is ENTIRELY different from ti-basic. If you want to know more about Z80 asm take this link:http://www.omnimaga.org/index.php?topic=2076.0
here you can download lessons. Before you get to that though I recommend to install TASM, not SPASM. You can find instructions on setting up TASM in this Guide:http://www.ticalc.org/pub/text/z80/
(^take the second file, 83pa28d.zip)
Now I HIGHLY recommend using the first link I gave you for learning, once you are through that guide take the second one. I also recommend you to get a EMULATOR. Like Ti-Flash debugger or WabitEmu.
If you have any questions feel free to ask! I'll gladly write a comprehensive setup guide for you!