can I convert an existing back up drive to a time machine drive?

The simple answer is no. A very complicated answer.....involving setting up separate partitions on the drive.....would be maybe, with no guarantee that the current data on the drive would be preserved.

If you decide to try this route, you will need to copy all of the existing data on the drive over to another drive for safekeeping. Then erase the drive and set up 2 separate partitions. After that is done, copy the data back to one of the partitions on the drive and use Time Machine for the other partition on the drive.

It's a lot of work, and frankly intimidating if you have not done something like this before.

Things would be tons easier.....and much safer for your data...... if you could simply add a separate drive and use that just for Time Machine.

if possible, you can partition it. If it is already in APFS vault format, you might be able to get away with just adding a volume.