Hi there Hayashi, Welcome :-)
glad you got an A500+, as with tracking, the more chipram the better :-D and practice makes perfect.
if you want to "kick it up a notch", download octamed soundstudio, and find a midi interface and cheap keyboard or midi controller. good for laying down baselines or riffs in real time, and then tweaking the timing in the tracker so it sounds right ;-)
a hard disk and spot of extra memory works wonders and will make working with your machine much nicer. keep an eye out on ebay or amibay for a GVP HD8 for A500. there are a myriad of others, but these seem to be popular in the 2nd hand market and can take 30pin simms for up to 8Mb ram, and don't seem to have a problem with most any hard disks, or scsi-ide and ide-compact flash adapters.
as for transfering iff files, they should fit on a 720kb formatted PC floppy disk easily, so that is seemingly the simplist way. i think OS2.x came with a PC floppy dosdriver? could be wrong. i know OS3.x did....
anyway, good luck, and have fun :-)