Can a song have different keys?

Do songs have to be in the same key?

If a song is in a certain key, can you add an instrument or vocal to the song / track as long as they are in the same scale of that key. So for example, if a song is in key D major, you can add any instrument/vocal as long as the key is in the D major scale, so E minor, B minor, G Major, A Major, etc, etc.

Can keys change in a song?

Changing the key of a piece of music is called transposing the music. Music in a major key can be transposed to any other major key; music in a minor key can be transposed to any other minor key. (Changing a piece from minor to major or vice-versa requires many more changes than simple transposition.)

Can a song have 2 key signatures?

Indeed you can play more than one time signature at a time - it's called a polymetric rhythm. Some bands make a career out of doing just that, like one of my all-time faves Meshuggah.