Yes you can access the tablet's internal memory from a PC using WinSCP. That's one of its great features.

Since you have the files over on your media card, you should be able to copy them over to /usr/share/dosbox directory. One question,...did you run emelfm2 while in root mode in XTerm? If not, try running emelfm2 in root mode and then copy the files. If this doesn't work and you still don't have access permissions, you can use emelfm2 to read and change directory permissions. In emelfm2, go to the /usr/share directory and tap on the dosbox directory. Press the menu key (the middle one on the left below the Escape key) and it will pull up a menu. Select Actions, and then select Change permissions. You'll see what the current permissions are for the directory with boxes you can check and uncheck to change the permissions.