Salt does not share much code with the other diagrams, this is really an independant stuff.
So you cannot define sprite as in other diagrams.
However, there is an undocumented feature that is close to sprite. Using << and >> you can define a sprite-like drawing and reusing it latter.
For example:
@startsalt
{
[X] checkbox|[] checkbox
() radio
(X) radio
This is a text|[This is my button]|This is another text
"A field"|"Another long Field"|[A button]
<<folder
............
.XXXXX......
.X...X......
.XXXXXXXXXX.
.X........X.
.X........X.
.X........X.
.X........X.
.XXXXXXXXXX.
............
>>|other|<<folder>>
^Droplist^
}
@endsalt
Maybe you can use it instead ?