When the iMac was new (1998), there were some USB-to-SCSI adapters. You may be able to locate one, but whether drivers exist that would allow use with a modern Mac is not at all certain. There are also hardware questions (connectors, power supply) to be solved. This is not a method I would recommend.
If the Macintosh IIsi is fully functional, you could use 1.44 MB floppies for transfers of smaller files. A modern Mac needs an external USB floppy drive. A Windows PC with a built-in floppy drive would have to use a special utility (such as TransMac) for the handling of Mac-formatted 1.44 MB disks. In both cases, do not expect to transfer applications, only data files.
It is also possible to transfer files via a serial null-modem connection. See the following discussion for some details:
https://discussions.apple.com/thread/4974860
A suitable Macintosh computer equipped with Ethernet can be used as an intermediary. For example, a PowerBook 520/520c/540/540c. A LocalTalk connection (by means of a Mac printer cable) is possible between the IIsi and a PB 5xx.
Jan