Is it impossible tu turn aluminum or brass tubing to for butt caps on a wood lathe?

Nothing's impossible, but some are more possible than others. If you just wanted to cutoff the length of a piece of tubing, you could probably use a (wood type) cutoff tool, and then file/sand it smooth. But to do any more complicated shaping, I don't think us humans have the fine control you'd need, since even hefty metal lathes take off pretty fine shavings comparted to wood shavings. Don't bother with gouges/skew chisels, but try a scraper and a very steady hand. You could probably also cut some simple decorative grooves, using a pointed scraper. I've now backed up my wood lathe with a small, 7x10, Grizzly metal lathe and that's a lot handier for many rodbuilding tasks.