JabRef can optionally mark all new entries added or imported to a database with your username.
You can disable or enable this feature by entering Preferences → General, and selecting/deselecting the line 'mark new entries with owner name'. You can also change the name used to mark your entries. The default name used is your user name. Finally, if an entry with a field owner is pasted or imported, the field is updated with your owner name if 'Overwrite' is checked.
The owner name is added in a field called 'owner', which by default is visible in the General fields tab in the entry editor.