Adding cue-notes staff

The term to look for is 'ossia'.

Either see https://musescore.org/en/node/46236
Or create it in an empty score, then use the snapshot/screenshot tool and add it back in as an image.

1. If you mean an extra staff on a instrument part then there are 2 ways to do it. Make another staff and hide all the empty measures. there are a few ways to hide measures, and you will probably have to use more than one to get the look you want.

2. The other way is to make a measure in its own file (or temporarily use the measure out of you existing score and delete it when done). The the image capture to take a picture, save it to a file and paste it into the score.

If you want it to be able to be played back you must use method 1. If it is strictly cosmetic use method 2, unless there are multiple spots in the score you need this done, then method 1 might make more sense. If you use method 2, wait until your score is almost finished so you can ensure the image is the correct size and position in your printed score/part.