Merging instrument part

It is possible, but a little tricky. If the trumpet does not use a mute anywhere in the song you can use the Trumpet Staff.

In the alto staff, swap voices 1 and 2. Select the also staff and use the inspector and choose voice 2. Copy this voice and paste it to the trumpet staff. You can then select all of the voices in the inspector so everything will work correctly in the future. In the trumpet staff put any text you like (probably something like

Trumpet
Alto

To show the top line is trumpet and the bottom line is Alto would be appropriate or even just an X that you can make invisible (select it a press V) if you don't have need for text. Right click this text, select Staff text properties and assign voice 2 to mute.

Finally open the mixer (press F10) and change the channel for must trumpet to alto and everything will be set up.

You can delete the alto staff since it is no longer needed.

If the trumpet uses a mute you will need to create a staff with a string instrument such as violin that has 3 channels. In the mixer assign channel 1 to trumpet, channel 2 (pizzicato) to mute trumpet and channel 3 (tremolo) to alto. When you make your text described above assign voice 2 to tremolo and it will play Alto.