OP, if you do not care about the partition/volume . . . then, I would try to erase all the partitions, and create a new one. Maybe that is what you mean by convert, but that is not the same thing--there is actually a way to convert HFS+ to APFS. It is non-destructive, though I think almost everyone would tell you to ensure you have a copy/backup of everything you need.
I am not sure this list is exhaustive, but to be clear, with Dev Beta 2 (should be the same as Public Beta 1), I have confirmed that AFPS, HFS+, Fat32, and exFat all work. I have had some trouble with things I have partitioned very carefully to a fair-the-well, though (like 10+ partitions). I am not yet sure why. Maybe they are standard for gpt partitions and iOS/iPadOS only support a certain number of partitions . . . maybe mbr standards are better supported or something. I'm not sure . . .