Lightroom's only capability of adding text on top of an image is through the watermark feature when you export or print; or using the Slideshow feature.
There is also a plug-in called Mogrify which can add text at export.
If you mean you somehow left a white space along side, top, bottom, left, right, an image the only way to add text to that white area is by using a program like Photoshop. Sure the Watermark option can place the watermark in any corner and can be rotated. But it sounds like you want to add text like you can do in PS or one of the other Adobe programs for doing design layout. Like InDesign or Illustrator.
It is possible to add text to an image in the Print module by selecting Photo Info in the right side panel and the option for Custom Text and then typing (or pasting) into the text box. However, the text will always be at the bottom of the cell and so can work well with an image in portrait aspect, but with a landscape aspect image the text will be along one of the short sides and oriented sideways. If you don't want to send the output to an in-house printer, in can be "printed to jpg".