Can I make my custom user fields available to other users?

Yes. Provide the other user with a DFA analysis file with the custom user field(s) that you want to share. When the other user opens that file, he should open the Custom Fields Definitions dialog (View->Custom User Fields) and check the box "Save as defaults" at the bottom of the dialog under the Current custom fields definitions. These fields will now be included with each new file. To remove a custom user field as a default, start a new file and delete the field in the Custom Fields Definitions dialog, and check the "Save as defaults" checkbox.