Can I partition drive without erasing it?

No. You can partition off free space without erasing the entire drive.

Disk Utility for macOS Sierra: Partition a physical disk using Disk Utility

With that said however, its always highly recommended to backup your data, when modifying the hard drive the data lives on. There's no guarantee everything will go smoothly and nothing will be lost, so its better to be prepared and have your data backed up in case the partitioning goes wrong.

You can only get the Sierra installer from the Mac App Store.

If your computer is currently running Sierra, you should be able to download High Sierra, and once the installer is running, point it to the new Partition to install it on without altering your current installation of Sierra.

Some details here:

How to install High Sierra on a partition

In that case, you will need to download the Sierra installer from the Mac App Store, and then make a bootable USB drive to install in on the new partition.

https://www.macworld.com/article/3092900/macs/how-to-create-a-bootable-macos-sie rra-installer-drive.html

Sierra download

If you are currently running Sierra, did you try the link I posted earlier? When the installer downloads, and opens, choose the new partition.

Sierra download

No, but you should back up your data first.

(156369)