As suggestion...
One can purchase an external HDD. For example, 1 TB Passport.
Connect this external HDD to the Win/PC system using USB port.
On the Win/PC machine, format the external drive as Win-NTFS. By default, it might already be properly formatted for Win/PC format.
Manually copy the Win/PC files to the external HDD. Before copying files, decide which files to keep and which files to leave behind.
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Connect external HDD (loaded with kept files) to the iMac using USB or FW Port (depending on the port types on the external HDD).
Copy files from HDD to the iMac (into a new DATA folder or ???? folder) on the iMac system.
Compare before and after files names - to ensure all files copied over.
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While external HDD is connected to iMac (usb or FW port), use iMac Disk Utility and re-format the external HDD. Format as Native MacOSX format (or what every its called).
While external HDD remains connected, configure iMac's Time Machine to "auto backup" the internal HDD to external HDD (that you just formatted.
In the end, one moved the "kept" files to the iMac and created local auto-backup configuration as well. 2 tasks completed with simple low cost external HDD drive. If wondering, this process worked for my iMac system….
Hope this helps…