TestLink

When I import test suites into a testplan from another project and test plan I see that the GUI assigns the test cases some ids. When I execute getTestCaseIdByName function I see that the function returns tc_external_id for some cases same as the old external id. Is there a way I can assign/preserve the same external test case ids across two different projects within two different test plans when I do import/export?
How are the testcase external id and test case id values mapped to the id values shown on the GUI?

provide the testprojectname and planname

external tc id is computed using a counter for each test project, and there is no provision to maitain same external id when importing on different test project (you can not know if external id is already used).