Someone wrote a patch to add it, but since it is not required for games it will never be added to official DOSBox as DOSBox is intended for games. Database programs are probably the worse thing you can try to run in DOSBox. Try vDos or vDosPlus instead. https://sourceforge.net/projects/vdosplus/

Try using a virtual machine like virtualbox or vmware, and install a Windows 2000 or XP 32 on it.

Long filenames are not part of standard DOS. Windows 95 introduced long filenames and added some DOS calls that allowed to use them, so you'll need a virtual machine. Windows 95, 98 and Me are badly supported on most virtual machines, so I'd recommend you that Windows 2000 or (better) XP.

http://www.cn-dos.net/msdos71/doslfns.htm

MS-DOS Player also seems to show initial support for LFNs..
http://takeda-toshiya.my.coocan.jp/msdos/

Personally, I'm sceptical when it comes to LFNs.
DOSBox emulates a clean 386/486 era PC with DOS 5.

Once LFNs get mainstream support, a big chaos might start.
Users will inevitable handle DOS files the same way they handle modern files.

Expect long batch files with spaces, special characters and case-sensitivness.
Just think of the already present umlaut issues (not translated to host OS), just in a bigger scale. 😉