Steps to replicate:
1. Use JIRA with the default value for jira.projectkey.pattern ( jira.projectkey.pattern= ([A-Z][A-Z]+) ).
2. In Step 1 of the Fogbugz importer ( Project Key Mappings (Step 1 of 6) ) specify an alphanumeric project key in Target project key for eg. ADMIN1
3. Complete the import process.
4. Navigate to Projects from the JIRA admin interface - new project is listed there.

Warning

If a user navigates to any of the imported issues, JIRA returns the following error:

ERROR
The project you are trying to view does not exist. Try browsing for projects.
If you think this message is wrong, please consult your administrators about getting the necessary permissions.

That is due to the fact that the project key doesn't match the pattern from jira.projectkey.pattern and user have just come across a different bug related to the project pattern from from JRA-15856.

Possible workaround
Modify your jira.projectkey.pattern from /WEB-INF/classes/jira-application.properties to suite your project key patten for e.g to allow alphanumeric project keys patten should be ([A-Z][A-Z0-9]+)