There are three approaches to consider.
1. Create the table and add the data as a single operation. You can modify TblFactory to do this.
2. Add the data to an existing table (issue: how do you know you have the correct number of rows/columns?). You can use XPath or TraversalUtil to navigate to the table.
3. Use content control databinding, with a repeat on the table row, to inject XML data automatically into the table cells. See www.opendope.org
for further info.
For approach 1 or 2, you need to know that a table cell's content list is accessed by tc.getEGBlockLevelElts() (in docx4j 2.6.0; in 2.7.0 you will be able to do tc.getContent() ).