Unfortunately, you cannot import the existing Windows partition into Boot Camp after you have already reinstalled macOS Sierra. Boot Camp only works with unallocated space on your hard drive, so you will need to recreate the Windows partition manually.
Here are the steps on how to do that:
- Back up your data. This is important because you will be erasing the existing Windows partition.
- Boot into macOS Recovery. To do this, hold down Command + R while restarting your iMac.
- Open Disk Utility.
- Select the Windows partition and click the Erase button.
- Choose ExFAT as the file system and click Erase.
- Quit Disk Utility and restart your iMac.
- Launch Boot Camp Assistant. This will guide you through the process of creating a new Windows partition and installing Windows.
Here are some additional tips for assigning more resources to the Windows partition:
- Use Boot Camp Assistant to resize the Windows partition. This will allow you to allocate more space to Windows.
- Use the Optimize Drives feature in Disk Utility to defragment your hard drive. This can free up space that you can then allocate to Windows.
- Use a third-party partition manager to resize the Windows partition. This can be a more advanced option, but it can give you more flexibility.
I hope this helps!