I,m gonna give you two alternatives:
1) with those IDE-USB adapters you can plug your old mac HD to your iMac and transfer everything
2) with a USB Floppy drive you can read on your iMac every 1.44 floppy recorded on your old mac
I own 4 generations of Macs:
1st) Mac Plus
and Mac Classic II
2nd) Performa 6200CD and Performa 6360
3rd) PowerMac G4 Digital Audio
4th) Macbook and Macbook Pro
I use to transfer files between them via Apple Talk, via SCSI external HD or Zip Drives. But, in your case, the two alternatives above are enough.
Later you must run a virtual machine on your iMac to open the content in your old HDD. Or, if you bring only files via FD, you should convert the files to updated file formats.
I hope I could help. I case of boubts, please contact me. Or send me your HDD and I transfer its content to a DVD-DL, so that you can have it ready to read on iMac.