The usual way is to use the sharing tools in VirtualBox (VB), to share a folder on the computer hard drive to the guest OS running within VB. But I'm not sure if that would work with Android, I've never tried. I've always downloaded APKs within Android-x86 itself, when I've used it.
Probably easiest and most certain way would be to use a USB stick. Copy the APKs to the USB stick, and then mount the USB stick within VB using VB's USB options. Then Android-x86 should just see it as an external storage device.
I know you can copy and paste text between the host and guest OS, but you can't copy and paste actual files AFAIK.