It's kind of difficult, because the Windows networking (SMB) that your OSX host uses (SMB 3.0
according to Apple) and your Win98 guest uses (SMB 1.0) are not talking to each other.
Your best option would be to create an ISO from your host (I prefer
Keka to do that) and mount it in your guest.
Just for completion, and if you don't want to use any 3rd party tools, VirtualBox provides its own ISO creation tool, but I think it's way too complicated for what you want to do. See ch. "9.36 VISO file format / RTIsoMaker" in your included User Manual PDF.
Moving them from Win98 -> 10.14, that's another story. And not VirtualBox related...