Just parking these instructions here so Google so people can add a Mac partition to an NTFS USB disk without losing data.
If you get a USB drive with NTFS, and try to resize & format a second partition for your Mac, you'll get an error from Disk Utility like this: "MediaKit reports not enough space on device for requested operation". Every bit of advice online says that the fix for this is to wipe the disk, which, for me, would wipe out the NTFS partition I wanted to keep.
The trick is simply to make an EFI partition >200 MB (I made a 260MB one), which you can do using Windows diskpart: https://www.easeus.com/partition-master/restore-repair-deleted-efi-boot-partition-in-windows-10-8-7.html
After you've made your EFI partition, format the rest of the space as an empty volume, and then the Mac will cope with it just fine.