Depends on how you're doing it.
You can setq(0,0) as your index to the current register count, and then something like:
[setq(0,inc(%q0))][setq(pack(%q0,36),Whatever.)]
If you're iterating through something you can do:
iter(list of a bunch of items,[do whatever][setq(pack(inum(),36),Whatever)])
But that'll run out of registers if you iterate through more than 35 (36 if you do dec(inum()) instead, since inum starts at 1).
That's the best I can do without more info about what you're trying to do.
--Stephen