4/4 + 1/8 as a time signature in a single measure

The best way in my opinion is to use the time signatures palette (shift + t) and create a 7/8 time signature. Apply this, right click it, choose time signature properties and change the display to 4/4. Elsewhere create a 1/8 time signature and use the capture tool with the DPI set to 360. Insert this picture next to the 4/4 and add a + sign from the master palette (or use staff text and adjust the font size) and place it between the 4/4 and 1/8 time signatures. You will need to make the adjustments on every instrument. Also, do this after all notes are entered on this system and put a system break before the line where this starts (if it's not the first) and on the last measure of this system. This will prevent MuseScore from changing the widths of the measures and making you readjust the + and picture.