I have a 1TB external hard drive with about 800gb data of ext4 format. Is there a way of converting it to ntfs format without having to format hence losing my data?
As far as I know, no. Being able to convert a FAT32 drive to NTFS is an exception, not a rule, and converting from one modern, journaled file system to another to another is implausible. For that matter, you can't convert from NTFS to another FS/
Any file system operation is tricky - I suppose you could shrink the EXT4 partition, then create a small NTFS partition, move some files, and repeat until everything is moved over and the EXT partition is gone, but one mistake and its all gone.
Backing up, reformatting and copying back the data is the smart choice.
You might also want to look into ext3/4 drivers for windows if its just a matter of access. I had reasonable luck in the past with them, but I haven't worked with them recently
It's not possible, but it certainly could be since file systems can be resized on the fly these days. That alone tells me it's entirely possible but it would probably never be recommended. 879 gigs isn't much, try managing like 11 TB.