I have created an excel template with macro to generate xml for importing requirements/test cases/test suites to testlink. This macro will generate two xml files - one for requirement (excel sheet name_Req.xml) and oen for test cases (excel sheet name_Tc.xml)

Link to Excel Sheet:
http://spreadsheets.google.com/ccc?key=0AjBCa6US6n8TdDF1Zk1hNGd1MGtHYnJhSVhTVUxoZFE&hl=en

Link to macro: (Saved as .txt. Please open the vba editor in excel, copy-paste the text and save)
http://docs.google.com/Doc?docid=0ATBCa6US6n8TZGRodmh3Y2hfM2N4dmZrYmc1&hl=en

Imports supported are as follows: (The table should start from Row 1, Column 1 as given)

Import Requirements:
The Excel format to be used is given below.
Spec Title: The Requirement Specification Title (This can be left empty, since this is only for mapping requirements with test cases. But, this column must be present.)
Document ID: This is the unique requirement reference number. (Must be unique when importing requirements. But, when importing test cases or test suites with requirements, multiple test cases can be mapped with single requirement. For more details, please see other import formats below.
Req Title: This is the title of the requirement. This is also unique. Each Document ID is mapped with a Req Title.
Description: This is the detailed description (Scope) of the requirement.
Import Test Cases:
The Excel format to be used is given below.
TC#: This is the Test Case Serial Number. It should start from 1 and be in sequence. This is also unique (even across test suites).
Name: This is the Test Case Name. This is also unique for a test case.
Summary: This is the Test Summary, a brief about the Test Scenario.
Steps: This section is for writing the steps for execution of the test case.
Expected Results: This is the results expected on executing the test case.
Import Test Suites with Test Cases:
The Excel format to be used is given below.
Name: Name of the Test Suite. Test Cases must be grouped under each Test Suite exactly as given below. This is unique.
Details: Details about the Test Suite.
Import Test Cases with Requirements:
The Excel format to be used is given below.
Import Test Cases with Custom fields:
The Excel format to be used is given below.
Import Test Cases with Custom Fields and Requirements:
The excel format to be used is given below.
Import Test Suite with requirements:
The excel format to be used is given below.