You can just for files by going to "terminal".
then enter on the command line:
ln <existing file> <link to be created>.
This wil create a hard link between the 2 entries.
You can see this by executing:
ls -l <existing file> <link to be created>
You'll see that the number of link (the first number) > 1, and the inode number is the same.
It is in MacOS not possible to create a hard link on directories (as far as I know).
And, how Dropbox handles the hard linked files I don't know. I would expect is doen not recognise the inode number and/or the number of links be the same, so it would create 2 different files, with exactly the same creation/change dates (as these are in the inode admin)