Hello illias,
if your card reader is not a USB device you can use the VBoxManage command line tool to link a virtual box disk image to a physical disk (SD card) on your system, by following the instructions
in section 9.7.1.1 here. For that you will need determine the device id of your SD Card. You should be able to look up the device id by opening the CMD shell in windows and entering:
wmic diskdrive list brief
Check the caption and model columns in the output to find your SD card. The column
DeviceID will show an id that usually has the form
\\.\PhysicalDriveX, with "X" being an integer.
However I think it would be much easier for you if you created a shared folder between your host and guest operating system and copy the files that way. Linked below is a short tutorial that shows you how to set up shared folders.