S3D will already boolean two separate STL files together when they overlap. So, for example, if you had a model of a cat and a model that was the text "CAT", if you overlapped those two parts, the software would print them as a single body. So I think all you need is an STL file for your part and then one for the text you want to add.
There is a cool add-in in Simplify3D that will convert a 2D image into a 3D solid. So for example, I made a quick image of the text for "CAT" (make sure to use black text on a white background). Then save this image as a PNG file (I attached mine to this post). Then go to Add-Ins --> Convert image to 3D. I just kept all the default settings, except changed the width to 100mm since that was closer to the size I wanted (depth scale 20, width 100 platform height 1, filter factor 1.0, enable invert depth profile)

- CAT.png (3.91 KiB) Viewed 9862 times
The result is pretty neat (see screenshot). So there might be other tools to convert text into a 3D model, but this worked great to me, plus you can use any font and styling you want!