May I suggest a way to add a new sprite to an existing metasprite without having to generate from selection?
Viewing post in List of suggestions
There are a number of ways i think you can do this.
A) using the right mouse button, drag a tile from the tileset onto the metasprite canvas. This tile is now added to your sprite at the position you drop it. This is great for when you want to add one sprite at a time.
B) Selecting one or more sprites on the list, you can click the "Dup" button just below. This duplicates selected sprites; whichafter you can configure their positions and Tile ID:s as desired.
C) Going to Menu > Metasprites > + Options > + merge to existing data; subsequent Make [ctrl+m] actions will merge the newly made metasprite with your old one. Some positional rearrangement is usually in order straight after.
(C) Sounds the closest to what you were describing, though I'm listing all of them in the order I tend to use them.
And just in case, were you looking for something different than one of those methods?