Lots more information, starting with your version of ID and your OS.
Are you trying to split a long block into multiple cells, and have the text reflow into the new cells (don't think that's going to happen witout manual intervention), or just add some more cells?
hmmn, not sure why not just merge cells or split columns as said but try this:
create a object style with 2 columns, then click in cell and insert an anchored object, assign the content to text and the object style to the style you created (eg two col ob). then tweak the setting as you need. paste the text in the ancored object 2 column tex frame.