Issues don't support custom fields, but you could create issue types as, for example, "IssueExt" and "IssueInt"
You can customize issue types by clicking the tools tab, then "built in fields.' Choose the "Issues" radio button and select "Type" to load the edit box with the currently-defined issue types. Add/change/delete as you like and click OK. Those types will then show up in the Issue dialog box's dropdown.
Alternatively, you could use the "External Reference" field to indicate the source of issues.
Hope this helps!
Ron
p.s. to change the fields you see in the issues tab, right-mouse click the heading section of that pane and select "choose columns"