Kent Cooper has written an improved text-to-mtext conversion routine based on code by Joe Burke. This version allows you to select any combination of text and mtext entities, and the selections will be joined into a single mtext object in AutoCAD. To use this routine, load it and use one of the following commands: 1MT0, 1MT1, or 1MT2. The first adds a space character between the selected strings, the second starts a new paragraph between the selections, and the third adds a blank line between the selections.
This routine requires that you select the objects one at a time, in the order they should be joined. If the first object is already mtext, its width property is retained. Kent has included more information about this routine as comments in the LISP file.